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.THE 

METALLOGRAPHY  AND 

HEAT  TREATMENT  OF  IRON 

AND  STEEL- 


ALBERT   SAUVEUR 

Professor  of  Mclnl/nrii,/  u/i</  Mct<ill<ii/rui>liy  in  Harvard  University  mid  Ihe 

Institute  of  Technology 


SECOND  EDITION  (THIRD  THOUSAND) 


SAUVEUR  AND   BOYLSTON 

METALLURGICAL  ENGINEERS 

CAMBRIDGE,  MASS.,  U.S.A. 

1916 


COPTRIGHT,   1910,    BY 

SAUVEUR  AND  BOYLSTON 


THE    rxiVKHHITY    PRKSS,    TAMBRIDRK,    C.  8.A. 


TO 

THE    MEMORY    OF 

&P  Jfattier 

I    REVERENTLY    AND    LOVINGLY 
DEDICATE    THIS    BOOK 


331170 


PREFACE  TO  THE  SECOND  EDITION 

THE  sale  in  less  than  three  years  of  two  impressions  of  the  first  edition  of  this 
book  justifies  the  author's  belief  expressed  at  the  time  that  there_was  a  need  in  the 
educational  and  industrial  world  of  a  treatise  on  the  metallography  of  iron  and  steel 
such  as  he  has  endeavored  to  supply.  Dealing  with  a  science  so  young,  and  there- 
fore still  in  full  growth,  it  was  to  be  expected  that  even  after  so  short  a  period  as 
three  years  there  would  be  need  of  a  revised  edition  that  recent  progress  might  be 
put  on  record  and  a  more  faithful  picture  offered  of  its  present  status.  The  neces- 
sary revisions  and  additions  have  been  made  to  the  best  of  the  author's  ability. 
Nearly  every  chapter  has  been  revised,  while  some  fifty  pages  of  new  text  have  been 
added  and  nearly  one  hundred  new  illustrations  used.  With  the  exception  of  the 
last  sixty-four  pages,  the  book  has  been  entirely  reset.  In  view  of  the  fact  that  so 
large  a  portion  of  this  work  is  devoted  to  the  study  of  the  heat  treatment  of  iron 
and  steel,  the  new  title,  namely,  "The  Metallography  and  Heat  Treatment  of  Iron 
and  Steel,"  appears  to  be  more  accurately  descriptive  of  its  contents. 

In  the  present  edition,  the  author  has  continued  to  follow  the  course  previously 
adopted  by  him,  to  utilize  the  best  illustrations  available  rather  than  to  use  his  own 
to  the  exclusion  of  others.  Too  many  books  purporting  to  be  treatises  on  certain 
subjects  are  merely  expositions  of  the  author's  views  to  the  belittling  of  the  opinions 
and  work  of  others.  Authors  of  treatises  should  be  just  and  impartial  and  should 
not  give  undue  prominence  to  their  own  views  and  opinions.  This,  obviously,  they 
owe  to  their  readers  and  to  the  public  in  general.  The  author  of  this  book  hopes 
that  he  has  succeeded  in  adhering  faithfully  to  that  belief  and,  if  he  has  failed  in  ever 
so  small  a  degree,  it  has  been  unintentional. 

Of  the  338  illustrations  reproduced  exclusive  of  illustrations  of  apparatus,  80 
were  prepared  by  the  author  himself,  82  by  others  in  his  laboratory,  and  25 
by  correspondence  course  students.  The  author  records  here  his  indebtedness 
to  the  following  gentlemen  for  illustrations  borrowed  from  them,  the  figures  in 
parentheses  indicating  the  number  in  each  case:  Messrs.  Andrews  (2),  Arnold  (5), 
Bayley  (1),  Belaiew  (5),  Benedicks  (2),  Brearley  (2),  Carpenter  and  Keeling  (2), 
Chappell  (1),  Coe  (1),  Sherard  Cowper-Coles  (1),  Desch  (7),  Edwards  (1),  Ewing  and 
Rosenhain  (2),  Franklin  (4),  Goerens  (8),  Guillet  (16),  Gulliver  (3),  Houghton  (1), 
Law  (7),  Levy  (1),  Longmuir  (2),  Maurer  (1),  Mellor  (1),  Osmond  (20),  Peirce  (4), 
Pulsifer  (1),  Roberts-Austen  (3),  Robin  (1),  Roland-Gosselin  (1),  Rosenhain  (7), 


vi  PREFACE  TO  THE  SECOND  EDITION 

Roozeboom  (1),  Ruff  (1),  Saladin  (2),  Sorby  (1),  Stead  (19),  Tschermak  (3),  Tscher- 
noff  (1),  Upton  (1),  Wittorff  (1),  Wiist  (3),  Ziegler  (1). 

He  also  takes  this  opportunity  of  expressing  his  appreciation  of  the  many  kind 
and  helpful  criticisms  and  suggestions  received  from  fellow-workers  and  other  cor- 
respondents. 

The  method  which  the  author  has  always  followed  in  his  teaching  of  metallography 
to  postpone  until  the  closing  chapters  the  study  of  the  equilibrium  diagram  and  of 
the  phase  rule  rather  than  to  introduce  the  subject  with  these  complex,  and,  there- 
fore, at  the  time,  forbidding,  considerations  has  been  adversely  criticized  by  a  few, 
but  on  the  whole  he  believes  that  it  has  won  out  and  he  is  more  convinced  than  ever 
that  it  is  the  most  effective  and  otherwise  satisfactory  method  to  follow. 

ALBERT  SAUVEUR. 
HARVARD  IJNIVERSITY, 
Cambridge,  Massachusetts, 
November  17,  1915. 


PREFACE   TO   THE   FIRST   EDITION 

WHILE  several  excellent  books  on  metallography  have  been_published  and  while 
numerous  papers  on  the  metallography  of  iron  and  steel  have  appeared  in  the 
scientific  and  technical  press,  a  well-balanced,  specific,  and  comprehensive  treatise 
on  the  subject  has  not  heretofore  been  written.  In  the  belief  that  there  is  a  real  and 
urgent  need  of  such  a  treatise  the  author  has  endeavored  to  supply  it,  craving  for  his 
effort  the  indulgent  criticism  of  his  readers.  He  offers  his  book  to  those  seeking  self- 
instruction  in  the  metallography  of  iron  and  steel,  their  special  needs  having  been 
carefully  considered  in  the  arrangement  of  the  lessons;  he  offers  it  to  teachers  and 
students  trusting  that  they  will  find  it  valuable  and  suggestive  as  a  text-book;  he 
offers  it  to  manufacturers  and  users  of  iron  and  steel  in  the  belief  that  he  has  given 
due  weight  to  the  practical  side  of  the  subject  and  has  avoided  discussions  of  ill- 
founded  or  purely  speculative  theories;  he  offers  it  to  the  general  reader  interested 
in  the  scientific  or  practical  features  of  the  metallography  of  iron  and  steel,  as  the 
language  used  should  be  readily  understood  by  those  lacking  specialized  knowledge 
of  the  subject;  he  offers  it  to  experts  in  the  hope  that  they  will  find  it  not  entirely 
devoid  of  original  thought,  original  treatment,  and  suggestiveness. 

In  the  matter  of  illustrations  and  especially  of  photomicrographs  the  author's  aim 
has  been  to  utilize  the  best  available,  using  his  own  or  those  taken  in  his  laboratory 
only  when  no  better  ones  have,  to  his  knowledge,  been  published  by  others.  The 
original  source  of  every  illustration  has  been  indicated  and  the  author  desires  to  ex- 
press his  indebtedness  to  the  following  writers,  the  figures  in  parenthesis  showing  the 
number  of  illustrations  from  each:  Andrews  (3),  Arnold  (7),  Bayley  (1),  Belaiew  (5), 
Brearley  (2),  Carpenter  and  Keeling  (l),Sherard  Cowper-Coles  (1),  Desch  (1),  Edwards 
(2),  Ewing  and  Rosenhain  (2),  Guillet  (18),  Gcerens  (9),  Gulliver  (2),  Hall  (1), 
Houghton  (1),  Kroll  (1),  Law  (8),  Levy  (1),  Longmuir  (2),  Matweieff  (1),  Maurer  (1), 
Mellor  (1),  Osmond  (17),  Roberts-Austen  (1),  Robin  (1),  Roland-Gosselin  (1),  Rosen- 
hain (2),  Saladin  (2),  Sorby  (1),  Stead  (13),  Tschermak  (3),  Tschernoff  (1),  Wiist  (5), 
Ziegler  (1).  All  illustrations  not  otherwise  inscribed  are  the  author's. 

The  author  cannot  refrain  from  expressing  here  the  sorrow  and  sense  of  personal 
loss  he  experienced  when  the  news  was  received,  while  this  book  was  passing  through 
the  press,  of  the  death  of  Floris  Osmond,  for  to  the  author,  as  no  doubt  to  many  others, 
Osmond's  work  and  Osmond's  life  have  been  an  inspiration.  Osmond  belonged  to 
that  admirable  class  of  French  scientists,  who,  like  Pasteur  and  Berthelot,  have  so 
lofty  a  conception  of  the  duty  of  the  scientist  that  they  give  to  the  world  the  fruit  of 


viii  PREFACE 

their  genius  and  of  their  untiring  labors  with  no  thought  of  monetary  return  or  even 
of  honorary  recognition.  If  Sorby  was  the  pioneer  of  metallography  and  Tschernoff 
its  father,  Osmond  has  been  its  torch-bearer  for  he,  more  than  any  other,  has  been  our 
guide.  While  he  is  no  longer  with  us,  his  light  will  long  continue  to  burn  and  to  show 
the  way  to  promising  and  productive  fields  of  research. 

The  author  desires  to  place  on  record  his  warm  appreciation  of  the  assistance  he 
received  from  Mr.  H.  M.  Boylston  in  passing  this  book  through  the  press,  and  also 
for  many  valuable  suggestions. 

ALBERT  SAUVETJR. 

HARVARD  UNIVERSITY, 

CAMBRIDGE,  MASSACHUSETTS, 
August  19,  1912. 


TABLE    OF   CONTENTS 

INTRODUCTION 

PAGE 

THE  INDUSTRIAL,  IMPORTANCE  OF  METALLOGRAPHY 1 

CHAPTER   I  — APPARATUS   FOR   THE   METALLOGRAPHIC   LABORATORY 

THE  MICROSCOPE 5 

The  stage 5 

Plain  stages 8 

Mechanical  stages 8 

Objectives 8 

Eye-pieces 8 

Iris  diaphragms 11 

Specimen  holders 11 

UNIVERSAL  METALLOSCOPE 14 

Electromagnetic  stage 15 

Templets  for  the  examination  of  small  specimens 15 

Support  of  non-magnetic  specimens 16 

Leveling-devices  of  stand  and  stage 16 

Motion  of  the  stage 16 

Mechanical  stage • 17 

Examination  of  transparent  objects 18 

ILLUMINATION  OF  THE  SAMPLES 18 

Vertical  magnifier 23 

SOURCES  OF  LIGHT  AND  CONDENSERS 23 

Condensers 26 

Monochromatic  light 27 

PHOTOMICROGRAPHIC  OUTFITS 27 

INVERTED  MICROSCOPES 31 

POLISHING  APPARATUS 34 

Hand  polishing  . 34 

Polishing  by  power 34 

Portable  polishing  motor 35 

PYROMETERS  AND  ELECTRIC  FURNACES 36 

Pyrometers 36 

Electric  furnaces 39 

CHAPTER  II  —  MANIPULATION 

Cutting  of  samples 40 

Polishing  by  hand 40 

Polishing  by  power .41 

Polishing  very  small  specimens 41 

Etching 42 

Nitric  acid  and  alcohol 42 

Picric  acid.    (Igevsky) 42 

Concentrated  nitric  acid.     (Sauveur) 42 

Sodium  picrate  etching  of  cementite 43 

Stead's  reagent  for  the  detection  of  phosphorus  segregation  in  iron  and  steel 43 

Heat-Tinting 44 

Sulphur  printing 44 

Etching  wrought  iron 45 

Etching  pearlitic  steel 46 

Etching  sorbitic  steel 46 

Etching  troostitic  steel 46 

Etching  martensitic  steel 46 

Etching  austenitic  steel 46 

ix 


x  TABLE  OF  CONTENTS 

PAGE 

Etching  alloy  steel 47 

Etching  cast  iron '.    .    .  47 

Etching  for  macrostructure 47 

Examination 47 

Photomicrography 48 

Exposure 49 

Diaphragms  and  shutters 50 

Monochromatic  light 50 

Photographic  plates 50 

Development 50 

Printing 50 


CHAPTER  III  —  APPARATUS   AND   MANIPULATION    (Continued) 

POLISHING  AND  POLISHING  MACHINES 51 

Polishing  small  specimens 61 

DEVELOPMENT  OP  THE  STRUCTURES 61 

Polishing  in  relief 61 

Polish-attack 62 

Etching 62 

Electrolytic  etching 64 

Hot  etching 64 

Washing  and  drying 64 

Preserving 64 

MOUNTING  AND  MOUNTING  DEVICES 65 

Plastic  mounting 65 

Leveling  stages     66 

METALLURGICAL  MICROSCOPE 67 

Le  Chatelier <>7 

Ernst  Leitz 67 

P.  F.  Dujardin 71 

C.  Reichert 71 

Robin     73 

Scientific  Materials  Co 73 

Martens      75 

Rosenhain 77 

Osmond 77 

Nachet 79 

Cornu-Charpy 

Watson  and  Sons      79 

Workshop  microscopes 81 

R.  and  J.  Beck 

F.  Koristka    .    .                       84 

Ph.  Pellin   ...                   84 

CarlZeiss 85 

Spencer  Lens  Co 

Bausch  and  Lomb  Optical  Co 85 


CHAPTER   IV  — PURE   METALS 

Microstructure 86 

Crystallization  .    .    . • 86 

Idiomprphic  crystals 

Allotrimorphic  crystals 

Crystallization  of  metals 87 

Grains  of  metals 89 

Crystalline  orientation  of  the  grains 

Cubic  crystallization  of  metals.  —  Etching  pits      90 

Summary 90 

The  amorphous  cement  theory  and  the  boundaries  of  the  crystalline  grains     .    .    . 

Straining  of  metals.  —  Slip  bands 92 

The  amorphous  cement  theory  and  the  straining  of  metals 

Twinning  and  twin  crystals 95 

Lines  of  Neumann 96 

Influence  of  mechanical  treatment 96 

Influence  of  thermal  treatment 96 

Amorphous  cement  theory  vs.  the  heat  treatment  of  pure  metals 97 

Impurities  .    .    .  • 97 


TABLE  OF  CONTENTS  XI 
CHAPTER   V  — PURE  IRON 

PAGE 

Microstructure 

Cubic  crystallization  of  iron •    •  103 

Ferrite : 104 

Allotropy  of  iron 

Solidification  and  crystallization  of  pure  iron      107 

Twinnings  and  Neumann  lines 109 

Strains  and  slip  bands 

Influence  of  mechanical  treatment 

Influence  of  thermal  treatment      110 

Crystallizing  properties  of  electrolytic  iron »  — t  ^ 

Influence  of  impurities 112 

CHAPTER  VI  — WROUGHT   IRON 

Chemical  composition      114 

Microstructure  of  longitudinal  section 114 

Microstructure  of  transverse  section 115 

Structural  differences  between  various  kinds  of  wrought  iron 116 

Chemical  composition  of  slag 116 

Microstructure  of  slag 116 

Influence  of  thermal  and  mechanical  treatments 117 

CHAPTER  VII  — LOW   CARBON   STEEL 

Normal  structure 118 

Grading  of  steels  vs.  carbon  content 118 

Low  carbon  steel  vs.  wrought  iron 118 

The  structure  of  low  carbon  steel      119 

Pearlite 120 

Free  ferrite 121 

Cementite      122 

CHAPTER   VIII  — MEDIUM   HIGH  AND   HIGH   CARBON   STEEL 

Medium  high  carbon  steel 124 

High  carbon  steel • .    .    .  126 

Eutectoid  steel      126 

Hyper-eutectoid  steel 127 

Free  cementite      129 

Hypo-  vs.  hyper-euteetoid  steel 129 

Etching  of  cementite 131 

Carbon  content  of  pearlite      131 

Structural  composition  of  steel 132 

Chemical  vs.  structural  composition 134 

Micro-test  for  determination  of  carbon  in  steel 135 

Physical  properties  of  the  constituents  of  steel 137 

Tenacity  of  steel  vs.  its  structural  composition 138 

Steel  of  maximum  strength 140 

Ductility  of  steel  vs.  its  structural  composition      141 

Diagram  showing  the  relation  between  the  tenacity  and  ductility  of  steel  and  its  carbon  content  142 

CHAPTER  IX  —  IMPURITIES   IN   STEEL 

Metallic  impurities 143 

Non-metallic  or  oxidized  impurities 143 

Metallic  vs.  non-metallic  impurities 143 

Gaseous  impurities 143 

Impurities  vs.  physical  properties  of  steel 143 

Silicon  in  steel       143 

Phosphorus  in  steel 144 

Sulphur  in  steel 145 

Sulphur  printing 148 

Manganese  in  steel 148 

Chemical  vs.  structural  composition 148 

Non-metallic  or  oxidized  impurities      150 

Segregation  of  impurities.  —  Ghosts 153 

Gaseous  impurities 157 

l 


xii  TABLE  OF  CONTENTS 

CHAPTER   X  — THE  THERMAL   CRITICAL   POINTS   OF   IRON   AND    STEEL 

I'AQE 

Point  of  recalescence 158 

Notation      159 

Critical  range.  —  Transformation  range 159 

Position  of  An  and  Aci 159 

Speed  of  cooling  and  heating  vs.  position  of  AI 161 

Temperature  from  which  cooling  begins  vs.  position  of  Ari 162 

Chemical  composition  vs.  position  of  Ai      162 

Upper  critical  points 

Thermal  critical  points  in  pure  iron 163 

Equilibrium  temperature  for  A3  and  A3.2 

Peculiarities  of  the  point  A2 164 

Thermal  critical  points  in  very  low  carbon  steel 164 

Thermal  critical  points  of  medium  high  carbon  steel 165 

Merging  of  As  and  Aj      165 

Thermal  critical  points  in  eutectoid  steel 165 

Merging  of  A3.2  and  Ai 165 

Factors  influencing  the  position  of  the  upper  points  As  and  A2 166 

Thermal  critical  points  in  hyper-eutectoid  steel 166 

Merging  of  As. 2.1  and  Acm 167 

Minor  critical  points 167 

Data  showing  the  position  of  the  critical  points 167 

Relative  quantities  of  heat  evolved  or  absorbed  at  the  critical  points      167 

Graphical  representation  of  the  position  and  magnitude  of  the  critical  points 169 

Determination  of  the  thermal  critical  points 169 

Cooling  and  heating  curves 169 

Use  of  neutral  bodies 173 

Additional  illustrations  of  cooling  curves 177 

Self-recording  pyrometers 178 

Other  methods  for  the  determination  of  the  critical  points 178 

Metallographic  method  for  the  determination  of  the  critical  points 179 

Calorimetric  method  for  the  determination  of  the  critical  points 179 

Thermo-electric  method  for  the  determination  of  the  critical  points 179 

Melting-points  method  for  the  determination  of  the  critical  points 179 

Magnetic  method  for  the  determination  of  the  critical  points 179 

Historical 180 

CHAPTER   XI  — THE   THERMAL   CRITICAL   POINTS   OF   IRON   AND    STEEL 

THEIR  CAUSES 

Causes  of  the  upper  points  As  and  A2  in  carbonless  iron 182 

Causes  of  the  upper  critical  points  A3  and  A2  in  low  carbon  steel 185 

Cause  of  the  point  A3.2 

Cause  of  the  point  Ai 189 

The  point  Ai  an  allotropic  point 

Pearlite  formation 190 

Cause  of  the  point  Acm 191 

Allotropy  of  cementite i92 

Cause  of  the  point  As.z.i  in  eutectoid  steel 

Cause  of  the  point  A3.2.i  in  hyper-eutectoid  steel 194 

Summary 

Another  view  of  the  allotropic  changes 1 96 

CHAPTER   XII  — THE   THERMAL   CRITICAL  POINTS   OF   IRON   AND    STEEL 

THEIR  EFFECTS 

Changes  at  A3 199 

Dilatation 198 

Electrical  conductivity 200 

Crvstallization 200 

Tensile  strength : 200 

Dissolving  power  for  carbon 

Structural  properties 

Other  properties 201 

Changes  at  A2 202 

Dilatation 202 

Magnetic  properties 203 

Crystallization 204 


TABLE  OF  CONTENTS  XU1 

PAGE 

Tensile  strength 204 

Dissolving  power  for  carbon 

Structural  properties 

Specific  heat ; 205 

Changes  at  As.» 205 

Changes  at  Ai 205 

Changes  at  As. 2.1 206 

Changes  at  Acm 206 

Structural  change  at  Ai  and  A3.2.i 206 

Prevailing  conditions  above  and  below  the  critical  range 206 

Properties  of  gamma,  beta,  and  alpha  iron  207 

CHAPTER   XIII  — CAST   STEEL 

Crystallization  of  steel 208 

Octahedric  crystallization  of  austenite     215 

Structure  of  cast  cutcctoid  steel 216 

Structure  of  cast  hypo-eutectoid  steel 216 

Structure  of  cast  eutectoid  vs.  cast  hypo-eutectoid  steel 217 

Structure  of  cast  hyper-eutectoid  steel 218 

Ingotism 220 

CHAPTER   XIV  — THE   MECHANICAL   TREATMENT    OF    STEEL 

Hot  working 221 

Finishing  temperatures 223 

Structure  of  hot  worked  eutectoid  steel 224 

Structure  of  hot  worked  hypo-eutectoid  steel     224 

Structure  of  hot  worked  hyper-eutectoid  steel 225 

Sorbite 225 

Hot  working  of  steel  vs.  its  critical  range 226 

Cold  working 227 

Mechanical  refining      229 

CHAPTER   XV  — THE   ANNEALING    OF   STEEL 

Purpose  of  annealing 231 

Nature  of  the  annealing  operation 231 

Heating  for  annealing      231 

Time  at  annealing  temperature      233 

Cooling  from  annealing  temperature 233 

Rate  of  cooling  vs.  carbon  content 234 

Rate  of  cooling  vs.  size  of  object 234 

Furnace  cooling  from  annealing  temperature 235 

Air  cooling  from  annealing  temperature 235 

Properties  of  sorbite 236 

Influence  of  maximum  temperature 236 

Influence  of  time  at  maximum  temperature 238 

Oil  and  water  quenching  from  annealing  temperature 239 

Double  annealing  treatment 240 

Annealing  eutectoid  steel 242 

Annealing  hypo-eutectoid  steel 243 

Annealing  hyper-eutectoid  steel 244 

Annealing  of  cold-worked  steel 246 

Annealing  steel  castings 248 

Rate  of  cooling  vs.  the  structure  of  steel     250 

Structure  vs.  heat  treatment 252 

Spheroidizing  of  cementite 252 

Varieties  of  pearlite 257 

Graphitizing  of  cementite 257 

Overheating 259 

Burnt  steel 259 

Crystalline  growth  of  austenite  above  the  critical  range 262 

Crystalline  growth  of  ferrite  below  the  critical  range 265 

Brittleness  of  low  carbon  steel 271 

Conclusions  regarding  the  annealing  of  steel 273 

CHAPTER   XVI  — THE  HARDENING   OF   STEEL 

Heating  for  hardening 274 

Cooling  for  hardening      275 

Structural  changes  on  hardening 276 


xiv  TABLE  OF  CONTENTS 

PASE 

Austenite 277 

Nature  of  austenite -<"' 

Occurrence  of  austenite 277 

Etching  of  austenite 

Structure  of  austenite 280 

Properties  of  austenite 281 

Mai-tensite 283 

Nature  of  martensite 283 

Occurrence  of  martensite 284 

Etching  of  martensite 284 

Structure  of  martensite 284 

Properties  of  martensite 285 

Troostite 

Nature  of  troostite •  -'*."> 

Occurrence  of  troostite 288 

Properties  of  troostite      289 

Etching  of  troostite 

Structure  of  troostite 289 

Sorbite 289 

Troosto-sorbite 290 

Hardenite • 291 

Rate  of  pooling  through  critical  range  vs.  structure  of  steel 

Are  the  transition  stages  distinct  constituents? 

Metarals  and  aggregates 293 

Hardening  eutectoid  steel 294 

Hardening  hyper-eutectoid  steel 

Hardening  hypo-eutectoid  steel     ' 

Steel  of  maximum  hardening  power 

Hardening  large  pieces 296 

Hardening  and  tempering  in  one  operation 297 

CHAPTER   XVII  — THE   TEMPERING    OF   HARDENED    STEEL 

Tempering  temperatures 

Tempering  colors      

Time  at  tempering  temperature 

Rate  of  cooling  from  tempering  temperature 

Hardening  and  tempering  combined 

Explanation  of  the  tempering  of  steel 

Tempering  austenitic  steels 

Tempering  martensitic  steel 

Tempering  troostitic  steel 

Tempering  troostito-martensitic  steel 

Tempering  troostito-sorbitic  steel 

Osmondite 

Structural  changes  on  slow  cooling,  quick  cooling,  and  reheating 

Microstructure  of  hardened  and  tempered  steel 

Carbon  condition  in  tempered  steel 

Decrease  of  hardness  on  tempering 

Heat  liberated  on  tempering 307 

CHAPTER   XVIII  —  THEORIES   OF   THE   HARDENING   OF   STEEL 

Retention  theories 

Solution  theories 

Beta  iron  or  allotropic  theory 

Alpha  iron  theory 

Amorphous  iron  theory 

Carbon  theories 

The  hardening  carbon  theory 

The  subcarbide  theory 

The  stress  theory      

Interstrain  theory 

Twinning  and  amorphous  iron  theory 

Tempering  and  the  retention  theories 

Tempering  and  tho  stress  theory 

Summary 313 

CHAPTER   XIX— THE   CEMENTATION   AND    CASE   HARDENING   OF    STEEL 

Composition  of  the  iron  and  steel  subjected  to  carburizing 

Carburizing  temperature 

Time  at  carburizing  temperature 316 


TABLE  OF  CONTENTS  XV 

PAGE 

Distribution  of  the  carbon 316 

Carburizing  materials 318 

Case  hardening  by  gas  under  pressure 319 

Mechanism  of  cementation 322 

Cooling  from  carburizing  temperature      324 

Heat  treatment  of  case  hardened  articles 324 

Tempering  case  hardened  steel 325 

CHAPTER   XX  — SPECIAL   STEELS 
GENERAL  CONSIDERATIONS 

Ternary  steels 326 

Influence  of  the  special  element  upon  the  location  of  the  critical  range     , _.  ._.   ^ 328 

Pearb'tic  steels 331 

Martensitic  steels 332 

Austenitic  (polyhedric)  steels 332 

Cementitic  (carbide)  steels 333 

Treatment  of  special  steels 333 

Treatment  of  pearlitic  steels 333 

Treatment  of  martensitic  steels 334 

Treatment  of  austenitic  steels 334 

Treatment  of  cementitic  steels 335 

Quaternary  steels 335 

CHAPTER   XXI  —  SPECIAL  STEELS 
CONSTITUTIONS,  PROPERTIES,  TREATMENT,  AND  USES  OF  MOST  IMPORTANT  TYPES 

Nk-kel  steel       336 

Manganese  steel 343 

Tungsten  steels 346 

Chrome  steels 349 

Vanadium  steels 349 

Molybdenum  steels 351 

Silicon  steels      . 352 

Chrome-nickel  steels 353 

§uaternary  vanadium  steels 353 

hrome-tungstcn  or  high-speed  steels 354 

CHAPTER   XXII  — CAST  IRON 

Solubility  of  carbon  in  iron 364 

Formation  of  combined  and  graphitic  carbon 366 

Cast  iron  containing  only  graphitic  carbon 366 

Cast  iron  containing  only  combined  carbon 369 

Cast  iron  containing  both  combined  and  graphitic  carbon 373 

Mottled  cast  iron     375 

Structural  composition  of  cast  iron 375 

Physical  properties  of  cast  iron  vs.  its  structural  composition 378 

Chilled  cast  iron  castings 378 

Cast  iron  of  euteetic  composition 379 

Eutectic  cast  iron  vs.  impurities 379 

The  strength  of  cast  iron  vs.  the  size  and  form  of  the  graphite  particles 380 

Eutectic  cast  iron  vs.  the  size  and  form  of  the  graphite  particles 380 

Silicon  and  the  rate  of  cooling  vs.  the  matrix  of  cast  iron  and  the  formation  of  graphite  .    .    .  380 

Cast  iron  of  maximum  strength 381 

Solidification  of  euteetic  cast  iron      381 

Solidification  of  hyper-eutectic  east  iron 382 

Solidification  of  hypo-eutectic  cast  iron 382 

The  graphitizing  of  cementite 384 

Graphitizing  of  hyper-eutectic  alloys 384 

Graphitizing  of  euteetic  alloys 384 

Graphitizing  of  hypo-eutectic  alloys 385 

CHAPTER   XX1I1  —  IMPURITIES   IN   CAST  IRON 

Silicon  in  cast  iron 3g6 

Sulphur  in  cast  iron 385 

Manganese  in  cast  iron 387 

Phosphorus  in  cast  iron 388 

Critical  points  of  cast  iron  containing  phosphorus 394 

Structural  composition  of  phosphoretic  cast  iron 394 

Chemical  vs.  structural  composition 395 

Other  impurities 397 


xvi  TABLE  OF  CONTENTS 

CHAPTER   XXIV  —  MALLEABLE   CAST  IRON 

PAGE 

Graphitizing  of  cementite 398 

Malleable  cast-iron  castings 398 

Original  castings 399 

Annealing  operation 400 

Packing  materials 400 

Annealing  for  malleablizing 401 

Annealing  for  "white  heart"  castings 401 

Annealing  for  "black  heart"  castings 402 

Cooling  from  annealing  temperature 406 

Gray  cast  iron  vs.  malleable  cast  iron 406 

CHAPTER   XXV  —  CONSTITUTION   OF   METALLIC   ALLOYS 

Solidification  of  pure  metals 407 

Solidification  of  binary  alloys  the  constituents  of  which  form  solid  solutions 409 

Fusibility  curves  of  binary  alloys  whose  component  metals  are  completely  soluble  in  each  other 

when  solid      411 

Binary  alloys  forming  definite  compounds  and  solid  solutions 

Binary  alloys  whose  component  metals  are  insoluble  in  each  other  in  the  solid  state 415 

Binary  alloys  whose  component  metals  are  partially  soluble  in  each  other  when  solid    ....  423 

CHAPTER   XXVI  — EQUILIBRIUM   DIAGRAM   OF  IRON-CARBON   ALLOYS 

Fusibility  curve  of  iron-carbon  alloys 420 

Structural  composition  of  iron-carbon  alloys  immediately  after  solidification 438 

Iron-graphite  fusibility  curve 434 

Combined  graphite-cementite  diagram 434 

Graphitizing  of  cementite 434 

Structure  of  iron-carbon  alloys  immediately  after  solidification 438 

Complete  equilibrium  diagram 439 

Historical 443 

Upton's  diagram 448 

Ruff's  diagram 449 

Wittorff's  diagram 450 

CHAPTER   XXVII  — THE   PHASE   RULE 

Enunciation  of  the  phase  rule 452 

Equilibrium 452 

Degrees  of  freedom 453 

Phases 454 

Components 454 

The  phase  rule  applied  to  alloys 454 

The  phase  rule  applied  to  pure  metals 455 

The  phase'rule  applied  to  binary  alloys 455 

The  phase  rule  applied  to  iron-carbon  alloys 457 

APPENDIX  —  NOMENCLATURE   OF  THE   MICROSCOPIC   CONSTITUENTS 

I.  GENERAL  PLAN 460 

II.  LIST  OF  MICKOSCOPJC  SUBSTANCES 461 

III.  DEFINITIONS  AND  DESCRIPTIONS 464 

Austenite ' 

Cementite 465 

Martensite 466 

Ferrite 466 

Osmondite 467 

Ferronite 468 

Hardenite 468 

Pearlite 468 

Graphite 469 

Troostite 470 

Sorbite 470 

Manganese  sulphide 471 

Ferrous  sulphide 471 

MISCELLANEOUS 471 

INDEX  473-487 


INTRODUCTION 

i 

THE  INDUSTRIAL   IMPORTANCE  OF  METALLOGRAPHY1 

Twenty  years  ago  the  science  of  metallography  was  practically  unknown  and  it 
is  only  within  the  last  fifteen  years  that  it  has  been  seriously  considered  by  metal 
manufacturers  and  consumers  as  a  valuable  method  of  testing  and  investigating. 
That  so  much  has  been  accomplished  in  so  short  a  time  is  highly  gratifying  to  the 
many  workers,  practical  or  scientific,  who  have  contributed  by  their  efforts  to  the 
progress  of  metallography. 

To  realize  the  practical  importance  of  metallography  it  should  be  borne  in  mind 
that  the  physical  properties  of  metals  and  alloys  — •  that  is,  those  properties  to  which 
these  substances  owe  thejr  exceptional  industrial  importance  —  are  much  more  closely 
related  to  their  proximate"  than  to  their  ultimate  composition,  and  that  microscopical 
examination  reveals,  in  part  at  least,  the  proximate  composition  of  metals  and  alloys, 
whereas  chemical  analysis  seldom  does  more  than  reveal  their  ultimate  composition. 

The  analytical  chemist  may  tell  us,  for  instance,  that  a  steel  which  he  has  analyzed 
contains  0.50  per  cent  of  carbon,  without  our  being  able  to  form  any  idea  as  to  its 
properties,  for  such  steel  may  have  a  tenacity  of  some  75,000  Ibs.  per  square  inch  or 
of  some  200,000  Ibs.,  a  ductility  represented  by  an  elongation  of  some  25  per  cent,  or 
practically  no  ductility  at  all;  it  may  be  so  hard  that  it  cannot  be  filed  or  so  soft  as 
to  be  easily  machined,  etc. 

The  metal  microscopist,  on  the  contrary,  on  examining  the  same  steel  will  report 
its  structural,  i.e.  its  proximate,  composition,  informing  us  that  it  contains,  for  in- 
stance, approximately  50  per  cent  of  ferrite  and  50  per  cent  of  pearlite,  and  we  know 
at  once  that  the  steel  is  fairly  soft,  ductile,  and  tenacious;  or  he  may  report  the 
presence  of  100  per  cent  of  martensite,  and  we  know  that  the  steel  is  extremely  hard, 
very  tenacious,  and  deprived  of  ductility. 

Which  of  the  two  reports  is  of  more  immediate  practical  value,  the  chemist's  or 
the  metallographist's?  Surely,  that  of  the  metallographist. 

Nor  is  it  only  in  the  domain  of  metals  that  we  find  such  close  relationship  between 
properties  and  proximate  composition,  for,  on  the  contrary,  it  is  quite  true  of  all 
substances.  How  many  organic  bodies,  for  instance,  have  practically  the  same  ulti- 
mate composition  and  still  are  totally  unlike  in  properties  because  of  their  different 
proximate  composition,  i.e.  different  grouping  and  association  of  their  ultimate  con- 
stituents. If  we  were  better  acquainted  with  the  proximate  composition  of  substances 
many  unexplained  facts  would  become  clear  to  us. 

Unfortunately  the  chemist  too  often  is  able  to  give  us  positive  information  in 
regard  to  the  proportion  of  the  ultimate  constituents  only,  his  reference  to  proximate 

1  Abstracted  from  a  paper  presented  at  the  Congress  of  Technology  at  the  fiftieth  anniversary 
of  the  granting  of  the  charter  of  the  Massachusetts  Institute  of  Technology,  April,  1911. 

1 


2     INTRODUCTION  —  THE  INDUSTRIAL   IMPORTANCE  OF  METALLOGRAPHY 

analysis  being  of  the  nature  of  speculation.  Ultimate  analysis  has  reached  a  high 
degree  of  perfection  in  regard  to  accuracy  as  well  as  to  speed  of  methods  and  analyt- 
ical chemists  have  built  up  a  marvelous  structure  calling  for  the  greatest  admiration, 
their  searching  methods  never  failing  to  lay  bare  the  ultimate  composition  of  sub- 
stances. But  how  much  darkness  still  surrounds  the  proximate  composition  of 
bodies  and  how  great  the  reward  awaiting  the  lifting  of  the  veil ! 

The  forceful  and  prophetic  writing  in  1890  of  Prof.  Henry  M.  Howe  naturally 
comes  to  mind.  Speaking  of  the  properties  and  constitution  of  steel,  Professor  Howe 
wrote: 

"If  these  views  be  correct,  then,  no  matter  how  accurate  and  extended  our  knowl- 
edge of  ultimate  composition,  and  how  vast  the  statistics  on  which  our  inferences  are 
based,  if  we  attempt  to  predict  mechanical  properties  from  them  accurately  we  be- 
come metallurgical  Wigginses  .  .  . 

"Ultimate  analysis  never  will,  proximate  analysis  may,  but  by  methods  which 
are  not  yet  even  guessed  at,  and  in  the  face  of  fearful  obstacles. 

"How  often  do  we  look  for  the  coming  of  the  master  mind  which  can  decipher  our 
undecipherable  results  and  solve  our  insoluble  equations,  while  if  we  will  but  rub  our 
own  dull  eyes  and  glance  from  the  petty  details  of  our  phenomena  to  their  great  out- 
lines their  meaning  stands  forth  unmistakably;  they  tell  us  that  we  have  followed 
false  clues  and  paths  which  lead  but  to  terminal  morasses.  In  vain  we  flounder  in 
the  sloughs  and  quagmires  at  the  foot  of  the  rugged  mountain  of  knowledge  seeking 
a  royal  road  to  its  summit.  If  we  are  to  climb,  it  must  be  by  the  precipitous  paths 
of  proximate  analysis,  and  the  sooner  we  are  armed  and  shod  for  the  ascent,  the  sooner 
we  devise  weapons  for  this  arduous  task,  the  better. 

"By  what  methods  ultimate  composition  is  to  be  determined  is  for  the  chemist 
rather  than  the  metallurgist  to  discover.  But,  if  we  may  take  a  leaf  from  lithology, 
if  we  can  sufficiently  comminute  our  metal  (ay,  there's  the  rub!)  by  observing  dif- 
ferences in  specific  gravity  (as  in  ore  dressing),  in  rate  of  solubility  under  rigidly  fixed 
conditions,  in  degree  of  attraction  by  the  magnet,  in  cleavage,  luster,  and  crystalline 
form  under  the  microscope,  in  readiness  of  oxidation  by  mixtures  of  gases  in  rigidly 
fixed  proportions,  we  may  learn  much. 

"Will  the  game  be  worth  the  candle?  Given  the  proximate  composition,  will  not 
the  mechanical  properties  of  the  metal  be  so  greatly  influenced  by  slight  and  unde- 
terminable changes  in  the  crystalline  form,  size,  and  arrangement  of  the  component 
minerals,  so  dependent  on  trifling  variations  in  manufacture  as  to  be  still  only  roughly 
deducible?" 

The  above  was  written  before  the  days  of  metallography,  or  at  least  when  metal- 
lography had  barely  appeared  in  the  metallurgical  sky  and  when  no  one  yet  had  fan- 
cied what  would  be  the  brilliant  career  of  the  newcomer.  Metallography  has  done 
much  to  supply  the  need  so  vividly  and  timely  depicted  by  Professor  Howe,  precisely 
because  by  lifting  a  corner  of  the  veil  hiding  from  our  view  the  proximate  composi- 
tion of  metals  and  alloys  it  has  thrown  a  flood  of  light  upon  the  real  constitution  of 
these  important  products.  Has  the  game  been  worth  the  candle?  Will  any  one 
hesitate  to  answer  in  the  affirmative  Professor  Howe's  question? 

Professor  Howe  with  his  usual  acumen  was  conscious  of  the  fact  that  proximate 
analysis,  while  likely  to  reveal  a  great  deal  more  of  the  constitution  of  metals  than 
ultimate  analysis  ever  could,  might  still  leave  us  in  such  ignorance  of  their  physical 
structure  as  to  throw  but  little  additional  light  upon  the  subject.  His  fear  was  cer- 


INTRODUCTION  —  THE   INDUSTRIAL   IMPORTANCE   OF   METALLOGRAPHY     3 

tainly  well  founded  and  surely  if  the  proximate  composition  had  been  obtained  by 
chemical  analysis  it  would  indeed  have  told  us  little  of  the  structure  or  anatomy  of 
the  metals.  In  the  domain  of  proximate  composition  chemistry  cannot  do  more  for 
the  metallurgist  than  it  does  for  the  physician. 

Invaluable  information  chemistry  does  give,  without  which  both  the  physician 
and  the  metallurgist  would  bo  in  utter  darkness,  but  it  throws  little  or  no  light  upon 
the  anatomy  of  living  or  inanimate  matter.  Its  very  methods  which  call  for  the  de- 
struction of  the  physical  structure  of  matter  show  how  incapable  it  is  to  render  assist- 
ance in  this,  our  great  need. 

The  parallel  drawn  here  between  metals  and  living  matter  is  not  fantastic.  It 
has  been  aptly  made  by  Osmond,  who  said  rightly  that  modern  science  was  treating 
the  industrial  metal  like  a  living  organism  and  that  we  were  led  to  study  its  anatomy, 
i.e.  its  physical  and  chemical  constitution;  its  biology,  i.e.  the  influence  exerted  upon 
its  constitution  by  the  various  treatments,  thermal  and  mechanical,  to  which  the 
metal  is  lawfully  subjected;  and  its  pathology,  i.e.  the  action  of  impurities  and  de- 
fective treatments  upon  its  normal  constitution. 

Fortunately  metallography  does  more  than  reveal  the  proximate  composition  of 
metals.  It  is  a  true  dissecting  method  which  lays  bare  their  anatomy  —  that  is,  the 
physical  grouping  of  the  proximate  constituents,  their  distribution,  relative  dimen- 
sions, etc.,  all  of  which  necessarily  affect  the  properties.  For  two  pieces  of  steel,  for 
instance,  might  have  exactly  the  same  proximate  composition  —  that  is,  might  con- 
tain, let  us  say,  the  same  proportion  of  pearlite  and  ferrite  and  still  differ  quite  a  little 
as  to  strength,  ductility,  etc.,  and  that  because  of  a  different  structural  arrangement 
of  the  two  proximate  constituents;  in  other  words,  because  of  unlike  anatomy. 

It  is  not  to  be  supposed  that  the  path  trodden  during  the  last  score  of  years  was 
at  all  times  smooth  and  free  from  obstacles.  Indeed,  the  truth  of  the  proverb  that 
there  is  no  royal  road  to  knowledge  was  constantly  and  forcibly  impressed  on  the 
mind  of  those  engaged  in  the  arduous  task  of  lifting  metallography  to  a  higher  level. 

Its  short  history  resembles  the  history  of  the  development  of  all  sciences.  At  the 
outset  a  mist  so  thick  surrounds  the  goal  that  only  the  most  courageous  and  better 
equipped  attempt  to  pierce  it  and  perchance  they  may  be  rewarded  by  a  gleam  of 
light.  This  gives  courage  to  others  and  the  new  recruits  add  strength  to  the  besieg- 
ing party.  Then  follows  the  well-known  attacking  methods  of  scientific  tactics  and 
strategy,  and  after  many  defeats  and  now  and  then  a  victorious  battle  the  goal  is  in 
sight,  but  only  in  sight  and  never  to  be  actually  reached,  for  in  our  way  stands  the 
great  universal  mystery  of  nature :  what  is  matter?  what  is  life? 

Nevertheless  there  is  reward  enough  for  the  scientist  in  the  feeling  that  he  has 
approached  the  goal,  that  he  has  secured  a  better  point  of  vantage  from  which  to 
contemplate  it.  The  game  was  worth  the  candle.  And  if  scientific  workers  must 
necessarily  fail  in  their  efforts  to  arrive  at  the  true  definition  of  matter,  whatever  be 
the  field  of  their  labor,  they  at  least  learn  a  great  deal  concerning  the  ways  of  matter, 
and  it  is  with  the  ways  of  matter  that  the  material  world  is  chiefly  concerned.  Hence 
the  usefulness  of  scientific  investigation,  hence  the  usefulness  of  metallography. 

Like  any  other  science  with  any  claim  to  commercial  recognition,  metallography 
has  had  first  to  withstand  the  attack  and  later  to  overcome  the  ill-will  and  reluctance 
of  the  so-called  "practical  man"  with  a  decided  contempt  for  anything  scientific. 
He  represents  the  industrial  philistine  clumsily  standing  in  the  way  of  scientific  ap- 
plications to  industrial  operations.  Fortunately,  while  his  interference  may  retard 


4     INTRODUCTION  —  THE   INDUSTRIAL   IMPORTANCE   OF   METALLOGRAPHY 

progress,  it  cannot  prevent  it.  Had  he  had  his  own  way  neither  the  testing  machine, 
nor  the  chemical  laboratory,  nor  the  metallographical  laboratory,  nor  the  pyrometer 
would  ever  have  been  introduced  in  iron  and  steel  works. 

Speaking  in  1904  of  the  practical  value  of  metallography  in  iron  and  steel  making, 
the  author  wrote  the  following,  which  it  may  not  be  out  of  place  to  reproduce  here: 
"History,  however,  must  repeat  itself,  and  the  evolution  of  the  metallographist  bids 
fair  to  be  an  exact  duplicate  of  the  evolution  of  the  iron  chemist;  the  same  landmarks 
indicate  his  course;  distrust,  reluctant  acceptance,  unreasonable  and  foolish  expecta- 
tion from  his  work,  disappointment  because  these  expectations  were  not  fulfilled  and 
finally  the  finding  of  his  proper  sphere  and  recognition  of  his  worth.  The  metal- 
lographist has  passed  through  the  first  three  stages  of  this  evolution,  is  emerging 
from  the  fourth,  and  entering  into  the  last.  For  so  young  a  candidate  to  recognition 
in  iron  and  steel  making  this  record  is  on  the  whole  very  creditable." 

We  may  say  to-day  that  he  has  definitely  entered  the  last  stage  and  that  the  ad- 
verse criticisms  still  heard  from  time  to  time,  generally  from  the  pen  or  mouth  of 
ignorant  persons,  are  like  the  desultory  firing  of  a  defeated  and  retreating  enemy. 

In  the  United  States  alone  the  microscope  is  in  daily  use  for  the  examination  of 
metals  and  alloys  in  more  than  two  hundred  laboratories  of  large  industrial  firms,1 
while  metallography  is  taught  in  practically  every  scientific  or  technical  school. 

A.  S. 

HARVARD  UNIVERSITY, 
February,  1912. 

1  As  the  second  edition  of  this  book  goes  to  press,  it  may  be  safely  affirmed  that  not  less  than 
four  hundred  industrial  laboratories  in  the  United  States  are  equipped  for  metallographical  testing. 


CHAPTER  I 

APPARATUS   FOR  THE  METALLOdRAPHIC  LAPjORATORY1 

Those  apparatus  which  the  author  has  found  most  satisfactory  are  described  in 
this  chapter  at  some  length;  others  more  briefly  in  Chapter  III. 

THE  MICROSCOPE 

While  any  good  microscope  of  the  ordinary  type,  substantially  built  and  provided 
with  a  satisfactory  fine  adjustment,  may  be  used  with  a  certain  degree  of  success  for 
the  examination  of  metals  and  alloys,  those  who  are  restricted  to  its  use  will  soon 
find  themselves  seriously  handicapped  in 'several  directions  and  unable  to  obtain  the 
desired  results.  The  following  considerations  will  make  this  clear. 

The  Stage.  —  Ordinary  microscope  stands  being  constructed  for  the  examination 
of  objects  by  transmitted  light,  i.e.  by  light  proceeding  from  below  the  stage  and 
passing  through  the  object  on  its  way  to  the  eye,  are  provided  with  fixed  stages. 
This,  however,  is  a  serious  objection  when  the  instrument  is  applied  to  the  examina- 
tion of  metals  and  other  opaque  objects,  which  must  necessarily  be  illuminated  by 
light  directed  upon  them  from  above  the  stage,  and  which  therefore  require  the  use  of 
an  "illuminator"  attached  to  the  objective  and  consequently  moving  with  it.  It  will 
be  readily  understood  that  it  is  of  considerable  importance  that  the  position  of  this 
illuminator,  and  therefore  of  the  objective  to  which  it  is  attached,  be  kept  constant, 
once  the  necessary  adjustments  are  effected,  since  any  change  in  its  position  would 
require  a  readjustment  of  the  source  of  light,  the  condensing  lenses,  diaphragm,  etc. 
To  that  effect  the  stage  should  be  provided  with  a  rack  and  pinion  motion  by  means 
of  which  the  coarse  focusing  at  least  may  be  done  (Fig.  1). 

This  rack  and  pinion  motion  of  the  stage,  moreover,  permits  of  a  much  greater 
working  distance,  allowing  plenty  of  room  for  the  insertion  of  the  illuminator  and 
nose-piece,  the  use  of  specimen  holders,  and  the  examination  of  bulky  specimens  with 
low-power  objectives.  In  the  microscope  illustrated  in  Figure  1,  the  working  distance 
measures  over  5  inches  as  against  4  inches  or  less  in  ordinary  stands. 

By  not  departing  more  than  necessary  from  the  usual  construction  of  microscopes, 
none  of  the  essential  features  required  for  the  examination  of  transparent  prepara- 
tions need  be  sacrificed,  and  the  full  efficiency  of  the  microscope  is  retained  for  such 
examination,  sub-stage  condensers,  polarizing  prisms,  etc.,  being  readily  attached 
when  needed.  The  possibility  of  applying  his  instrument  to  all  kinds  of  microscopical 
work  with  equally  satisfactory  results  should  appeal  strongly  to  the  metallographist, 
for  there  is  no  laboratory  where,  occasionally  at  least,  examination  of  transparent 
objects  is  not  desirable  or  even  imperative. 

1  Abstracted  in  part  from  papers  by  the  author  on  "Apparatus  for  the  Microscopical  Examina- 
tion of  Metals,"  American  Society  for  Testing  Materials,  Vol.  X,  1910;  and  "The  Universal  Metal- 
loscope,"  American  Institute  of  Mining  Knginoprs,  June,  1911. 


6        CHAPTER  I  — APPARATUS   FOR  THE   METALLOGRAPHIC   LABORATORY 


Fig.  1.  —  Metallurgical  microscope,  eye-piece,  vertical  illuminator,  objective, 
magnetic  specimen  holder,  and  mechanical  stage.     (0.5  actual  size.) 


CHAPTER   I  — APPARATUS   FOR  THE   METALLOGRAPHIC   LABORATORY         7 


Fig.  2.  —  Student  microscope.     (0.5  actual  size.) 


8        CHAPTER  I  — APPARATUS  FOR  THE   METALLOGRAPHIC  LABORATORY 

Less  expensive  but  satisfactory  microscopes  are  shown  in  Figures  2  and  3.  The 
latter  illustration  includes  an  auxiliary  tube  inserted  between  the  objective  and  illu- 
minator for  the  examination  of  large  samples  which  may  be  placed  on  the  base  of  the 
microscope  or  supported  in  some  other  suitable  way  below  the  stage. 

(a)  Plain  Stages.  —  While  a  mechanical  stage  adds  greatly  to  the  convenience  of 
the  manipulations,  a  plain  stage  may  be  used  with  satisfactory  results.  It  should  be 
provided  with  strong  clips  to  hold  in  place  the  specimen  holders  soon  to  be  described, 
and  should  preferably  be  circular  and  revolving  (Fig.  4).  When  provided  with  cen- 
tering screws  like  the  stage  of  the  stand  illustrated  in  Figure  1,  the  object  may  be 
moved  gently  while  under  examination,  a  very  desirable  feature  especially  when 
using  high-power  objectives,  in  which  case  the  moving  of  the  object  entirely  by  hand 
is  very  jerky.  In  order  to  derive  the  full  benefit  of  the  use  of  the  magnetic  holder 
described  later,  the  central  opening  of  the  stage  should  not  be  less  than  1%  inches  in 
diameter. 

(6)  Mechanical  Stages.  —  The  great  superiority  of  a  mechanical  stage  permitting, 
as  it  does,  a  systematic  examination  of  the  object  over  its  entire  surface,  need  not  be 
insisted  upon.  In  connection  with  the  magnetic  holder  it  makes  it  possible,  moreover, 
to  examine  repeatedly  and  at  any  time  the  same  spot  of  any  specimen,  as  will  soon  be 
explained.  The  mechanical  stage  illustrated  in  Figure  5  has  been  especially  designed 
to  fit  the  metallurgical  microscope  (Fig.  1),  and  is  very  readily  substituted  for  the 
plain  stage.  The  central  opening  measures  1%  inches  in  diameter,  permitting  the 
convenient  use  of  the  magnetic  holder. 

Objectives.  —  Ordinary  achromatic  objectives  give  satisfactory  results.  They 
should,  however,  be  corrected  for  uncovered  objects,  as  the  placing  of  cover  glasses 
over  bright  metallic  surfaces  is  accompanied  by  light  reflection  causing  loss  of  clear- 
ness and  definition. 

Some  believe  that  the  objectives  should  be  provided  with  short  mounts  so  as  to 
bring  the  reflector  of  the  vertical  illuminator  as  near  the  back  lens  of  the  objective  as 
possible,  and  thus,  in  their  opinion,  materially  decreasing  the  amount  of  glare  caused 
by  the  reflection  of  light  by  the  lenses  of  the  objectives.  They  are  advisable  only 
when  a  prism  vertical  illuminator  is  used.  Three  objectives,  one  of  low,  one  of  medium, 
and  one  of  high  power,  will  generally  suffice  for  metallographic  work.  The  following 
focal  lengths  are  recommended:  32-mm.  or  IJ^-in.,  16-mm.  or  -r-'s-in.,  and  4-mm.  or 
J^-in.  These  objectives  are  shown  in  Figure  6.  The  32-mm.  objective  is  provided 
with  a  society  screw  at  its  lower  end  in  order  that  the  vertical  illuminator  may  be 
inserted  between  the  objective  and  the  object,  this  being  desirable  with  very  low- 
power  lenses.  In  case  higher  power  is  needed,  a  1.9-mm.  or  ^-in.  oil  immersion  ob- 
jective will  be  found  very  satisfactory.  When  a  very  low-power  lens  is  required,  as 
for  instance  in  the  examination  of  fractures  or  of  very  coarse  structures,  a  48-mm.  or 
2-in.  objective  will  give  good  results.  It  is  suggested  that  it  be  provided  at  its  lower 
end  with  a  society  screw  to  permit  the  attachment  of  the  vertical  illuminator,  which 
in  the  case  of  such  low-power  lenses  should  be  placed  between  the  object  and  the  ob- 
jective, as  explained  later. 

Eye-Pieces.  —  With  achromatic  objectives  ordinary  Huygenian  eye-pieces  are 
used.  Two  eye-pieces,  respectively  of  1-in.  and  2-in.  focal  length,  will  generally  cover 
the  range  of  magnification  needed. 

For  the  taking  of  photomicrographs,  projection  eye-pieces  are  said  to  possess 
some  superiority,  especially  when  high-power  objectives  are  used,  as  they  then  yield 


CHAPTKR   I  — APPARATUS  FOR  THE   METALLOGRAPHIC   LABORATORY       9 


Fig.  3.  —  Student  microscope  fitted  with  auxiliary  tube. 


10      CHAPTER  I  — APPARATUS  FOR  THE   METALLOGRAPHIC  LABORATORY 


Fiu;.  4.  —  Plain  revolving  stage,  magnetic  specimen 
holder,  and  specimen. 


Fig.  5.  —  Mechanical  stage  to  fit  metallurgical  microscope, 
magnetic  specimen  holder,  and  specimen. 


CHAPTER   I  — APPARATUS   FOR   THE   METALLOGRAPHIC   LABORATORY       11 

flatter  and  more  sharply  defined  images.    The  Zeiss  projection  eye-piece  No.  2  is  very 
satisfactory. 

Iris  Diaphragms.  —  Iris  diaphragms  are  sometimes  inserted  between  the  objec- 
tives and  the  illuminator  so  as  to  control  the  size  of  the  pencil  of  light  proceeding 
from  the  object,  with  a  view  of  securing  sharper  definition.  Their  use  in  that  posi- 
tion, however,  is  of  doubtful  value,  as  it  may  cause  some  distortion  of  the  image.  It 
seems  preferable  to  place  the  iris  diaphragm  between  the  source  of  light  and  the 
illuminator,  thus  regulating  the  amount  of  light  entering  the  latter.  When  placed  be- 


Fig.  6.  —  Achromatic  objectives. 

tween  the  objective  and  the  illuminator  it  increases,  moreover,  their  distance  apart, 
which  we  have  seen  to  be  objectionable.  If  a  diaphragm  must  be  attached  to  the 
microscope,  it  is  better  to  place  it  between  the  tube  nose  and  the  illuminator.  When 
using  low-power  lenses  it  might  also  be  screwed  to  the  lower  end  of  the  objective,  thus 
controlling  the  light  returned  by  the  object  before  entering  the  objective. 

Specimen  Holders.  — •  In  order  to  examine  a  piece  of  metal  under  the  microscope, 
it  is  of  course  necessary  that  the  polished  and  otherwise  prepared  surface  be  held  in 
a  plane  accurately  perpendicular  to  the  optical  axis  of  the  instrument.  This  may  be 


I 


Fig.  7.  —  Specimen  holder. 


accomplished  by  so  shaping  the  sample  that  it  will  have  two  sides  exactly  parallel, 
and  preparing  one  of  them  for  microscopical  examination.  This  operation,  however, 
is  at  best  tedious  and  laborious,  and  metallographists  have  endeavored  to  replace  it 
by  the  use  of  more  or  less  ingenious  devices  for  holding  the  specimens  in  the  proper 
position.  Some  embed  their  samples  in  wax  or  in  some  other  plastic  material,  while 
others  have  recourse  to  stages  provided  with  special  leveling  devices. 

The  simple  holder  shown  in  Figure  7  gives  better  satisfaction,  requiring  no  mount- 
ing whatever  of  the  samples.    The  specimen,  no  matter  how  irregular  in  shape,  is 


12      CHAPTER   I  — APPARATUS   FOR   THE   METALLOGRAPHIC   LABORATORY 

held  firmly  in  place  by  a  rubber  band  and  the  holder  placed  on  the  stage  like  an 
ordinary  slide.  If  the  correction  of  the  objective  demands  it,  a  cover  glass  may  be 
inserted  between  the  sample  and  the  holder.  It  will  be  apparent  that  the  required 
manipulations  are  very  simple  and  quickly  performed. 

In  the  case  of  specimens  smaller  than  the  opening  of  the  holder,  however,  the  use 
of  a  cover  glass  is  necessary  to  hold  them  in  place.  This  is  objectionable,  at  least 
when  using  high-power  objectives,  which  should  be  corrected  for  uncovered  objects. 
To  overcome  this  difficulty,  a  little  templet  may  be  used  having  a  triangular  opening 
and  inserted  between  the  specimen  and  the  holder  (Fig.  8).  This  templet  is  made 
very  thin  so  as  to  permit  the  use  of  high-power  objectives,  which  must  be  brought 
very  close  indeed  to  the  object.  It  will  also  be  noticed  that  one  side  of  the  upper  part 


Fig.  8.  —  (a)  Specimen  holder  and  large  specimen. 

(6)  Specimen  holder,  templet,  and  small  specimen. 


of  the  holder  has  been  removed,  exposing  to  view  a  larger  portion  of  the  sample  and 
permitting  a  more  ready  approach  of  high-power  objectives.  Large  samples  are,  of 
course,  placed  in  the  holder  without  any  templet. 

A  still  simpler  and  more  effective  device  can  be  used  to  hold  in  place  samples  of 
iron  and  steel  and  other  magnetic  substances.  The  device  consists  of  a  V-shaped 
permanent  magnet  of  special  steel  about  1  inch  wide  and  2J/£  inches  long  (Fig.  9). 
This  little  magnet  is  placed  on  the  stage  of  the  microscope  like  an  ordinary  glass  slide 
(Figs.  4  and  5)  and  the  samples  to  be  examined  suspended  to  it  from  below,  being  held 
in  place  by  the  attraction  of  both  poles.  Small  samples  are  suspended  near  the  small 
end  of  the  V-shaped  opening,  while  larger  ones  are  placed  nearer  the  wider  end  of  the 
opening.  This  holder,  therefore,  is  universal  in  its  application  within  the  limits  of 
samples  of  suitable  size  for  microscopical  examination.  Its  beveled  edges  make  its 
use  possible  with  high-power  objectives  and  small  specimens.  If  the  opening  of  the 
stage  be  sufficiently  large,  say  1J4  inches  or  more  in  diameter,  the  magnet  may  be 


CHAPTER   I  — APPARATUS   FOR   THE    METALLOGRAPHIC   LABORATORY      13 

kept  permanently  on  the  stage,  as  the  samples  may  then  be  readily  removed  or  at- 
tached to  the  magnet  with  the  fingers  from  below  the  stage.  This  adds  so  much  to 
the  convenience  of  this  little  device  that  it  is  strongly  urged,  in  case  the  central  aper- 
ture of  the  stage  is  too  small,  to  have  it  suitably  enlarged.  The  magnet  is  kept  in 
place,  like  any  glass  slide,  by  the  clips  of  the  microscope  and,  also  like  any  glass  slide, 
may  be  moved  about  for  the  inspection  of  the  different  parts  of  the  specimen.  The 
side  of  the  magnet  resting  on  the  stage  having  been  ground  perfectly  flat,  it  will  be 


Sec  1 1  or? 
on   fl-  ff 


Fig.  9.  —  Magnetic  specimen  holder  with  large  and  small  specimens. 

evident  that  the  surface  of  the  sample  under  examination  will  always  be  accurately 
in  the  proper  position,  permitting  the  use  of  high-power  objectives  without  fear  of 
difficulty  arising  from  ever  so  slight  an  inclination  of  the  sample. 

When  used  in  connection  with  a  mechanical  stage  (Fig.  5)  the  convenience  of  this 
little  holder  becomes  still  more  apparent  and  its  usefulness  is  further  increased.  It 
then  affords,  moreover,  a  ready  means  for  the  repeated  examination  of  the  same  spot 
of  any  sample  at  any  time.  To  that  effect  the  holder  is  laid  upon  the  prepared  surface 


Fig.  10.  —  (a)  Magnetic  specimen  holder. 
(6)  Scratched  specimen. 


and  two  scratches  made  by  drawing  a  needle  across  the  specimen  along  the  sides  of 
the  Y-shaped  opening,  as  shown  in  Figure  10.  When  it  is  desired  to  examine  the  sam- 
ple, the  latter  is  suspended  to  the  magnet  so  that  the  needle  markings  coincide  closely 
with  the  sides  of  the  magnet  opening,  in  this  way  securing  a  permanent  position  for 
the  sample.  The  position  of  the  magnet  itself  is  controlled,  in  the  usual  way,  by  means 
of  the  graduating  devices  of  the  mechanical  stage. 

Finally,  by  placing  the  sample  below  the  stage  and  bringing  the  prepared  surface 


14      CHAPTER    I  — APPARATUS   FOR   THE   METALLOGRAPHIC   LABORATORY 

on  a  level  with  the  stage,  considerably  greater  working  distance  is  secured,  a  gain 
which  has  its  importance. 

Universal  Metalloscope.  —  The  instrument  shown  in  Figures  11  to  15  was  devised 
especially  for  the  ready  examination  of  large  iron  and  steel  samples,  but  it  will  be  ap- 
parent that  it  can  be  used  with  equally  satisfactory  results  for  small  samples  both 
opaque  and  transparent. 

Owing  to  the  fact,  however,  that  the  microscope  proper  and  the  stage  are  separate 
parts,  it  is  absolutely  necessary,  especially  for  the  use  of  high-power  objectives,  that 
the  apparatus  be  placed  on  a  support  free  from  vibration,  such  for  instance  as  a  suit- 


Fig.  It.  — Universal  metalloscopc:  stand,  eye-piece,  vertical  illuminator,  objec- 
tive, electromagnetic  stage,  and  rail  section. 

ably  constructed  concrete  pier.  With  that  precaution  taken,  excellent  results  are 
obtained. 

The  microscope  stand  proper  consists  of  a  microscope  tube,  provided  with  both 
coarse  and  fine  adjustments,  and  with  a  draw  tube,  rigidly  mounted  on  a  bar  sup- 
ported at  both  ends  on  substantial  and  firm  cast-iron  legs.1  The  height  between  the 
table  and  the  under  side  of  the  supporting  bar  is  5  inches  and  the  distance  between 
the  supporting  legs  12  inches. 

This  arrangement  affords  free  space  below  the  objective  for  the  examination  of 
large  specimens  of  metals,  such  as  full  rail  sections,  without  detracting  in  the  least 
from  the  value  of  the  instrument  when  applied  to  the  examination  of  the  usual  small 
specimens,  as  explained  later.  Many  metal  microscopists  frequently  have  to  examine 

1  In  a  more  recent  model  the  supporting  bar  is  mounted  on  three  legs,  permitting  the  ready  lev- 
eling of  the  instrument. 


CHAPTER   I  — APPARATUS   FOR   THE   METALLOGRAPHIC   LABORATORY      15 

bulky  specimens,  and  this  is  altogether  impossible  with  the  ordinary  microscopes  as 
well  as  with  the  special  metallurgical  microscopes  which  have  been  designed  and  de- 
scribed from  time  to  time. 

Recourse  must  be  had  to  all  sorts  of  makeshifts  for  the  proper  support  of  large 
specimens,  or,  more  often,  the  microscopist  gives  up  the  attempt  altogether,  or  else 
resigns  himself  to  the  cutting  of  the  bulky  samples  into  small  pieces  to  be  laboriously 
polished  and  separately  examined. 

It  is  believed  that  an  instrument  permitting  the  examination  of  large  as  well  as 
of  small  specimens  with  equal  ease  and  accuracy  will  be  welcomeoTby  metallographists, 
and  that  it  will  lead  to  more  frequent  examinations  of  full  sections  of  metal  imple- 
ments, a  departure  which  should  bring  fruitful  results. 

Electromagnetic  Stage.  —  The  perplexing  question  of  the  proper  support  for  mi- 
croscopical examination  of  iron  and  steel  specimens  of  all  sizes  and  shapes  has  been 


-rf 


Fig.  12.  —  (A)  Electromagnetic  stage  and  rail  section.  (B)  Electromag- 
netic stage,  templet,  and  medium-size  specimen.  (C)  Electromag- 
netic stage,  two  templets,  and  small  specimen. 

effectively  solved  by  the  use  of  the  electromagnetic  stage  illustrated  in  Figure  12. 
This  stage  consists  of  a  steel  plate  7  by  14  inches  having  a  V-shaped  opening,  and 
converted  into  a  powerful  electromagnet  by  means  of  two  bobbins  with  solenoids 
surrounding  the  arms  of  the  steel  plate,  as  clearly  shown  in  the  illustration.  Elec- 
trical connection  is  readily  made  with  any  suitable  current,  and  the  use  of  an  incan- 
descent lamp  in  series  provides  in  a  simple  way  the  necessary  outside  resistance  to 
prevent  heating  of  the  solenoids.  Large  specimens  of  iron  and  steel,  such  as  rail  sec- 
tions, A,  Figure  12,  are  firmly  held  in  an  accurate  position  by  the  attraction  of  the 
magnetic  stage,  the  extremities  of  the  flange  only  and  a  narrow  space  on  each  side  of 
the  head  being  hidden  from  view.  The  size  and  shape  of  the  stage-opening  make 
possible  the  ready  support  of  specimens  measuring  from  2  to  6  inches  in  their  greatest 
dimension. 

Templets  for  the  Examination  of  Small  Specimens.  —  For  the  examination  of  iron 
and  steel  samples  from  2  inches  in  length  down  to  the  smallest  dimensions,  a  steel 
templet,  also  with  a  V-shaped  opening,  is  placed  on  the  stage,  shown  at  B,  Figure  12. 
This  templet  through  its  contact  with  the  stage  becomes  strongly  magnetized  and  the 
specimens  to  be  examined  are  suspended  to  it. 

For  the  examination  of  very  small  specimens  with  high-power  lenses  the  thickness 


16      CHAPTER   I  — APPARATUS   FOR   THE   METALLOGRAPHIC   LABORATORY 

of  this  templet  would  prevent  the  necessary  close  approach  of  the  objective.  To 
make  this  approach  possible  a  very  thin  steel  templet  (not  exceeding  0.01  inch  thick) 
is  used,  shown  at  C,  Figure  12,  which  makes  possible  actual  contact  between  a  high- 
power  objective  and  the  smallest  specimen. 

Support  of  Non-Magnetic  Specimens.  —  For  the  support  of  non-magnetic  speci- 
mens, such  as  non-ferrous  metals,  rocks,  cement,  etc.,  a  very  simple  device  is  provided, 
consisting  of  two  crossbars  and  rubber  bands,  which  is  readily  attached  to  the  stage 
and  by  means  of  which  the  non-magnetic  specimens,  as  well  as  the  templets  when 
needed,  are  firmly  held  in  place  regardless  of  their  size  or  shape. 

Leveling-Devices  of  Stand  and  Stage.  —  It  is,  of  course,  essential,  especially  when 
using  high-power  objectives,  that  the  optical  axis  of  the  microscope  be  accurately 
perpendicular  to  the  surface  under  examination.  To  secure  this  result  both  the  stand 


Fig.  13.  —  Back  leg  of  electromagnetic  stage  and  sliding  plate. 

and  the  stage  are  provided  with  leveling-screws,  as  shown  in  Figure  11.  For  leveling 
the  stage  a  small  spirit-level  may  be  placed  upon  it,  or  better,  upon  the  sample  under 
examination,  and  the  necessary  adjustment  quickly  made.  For  leveling  the  micro- 
scope stand  the  eye-piece  should  be  removed,  the  small  level  placed  on  top  of  the  tube, 
and  the  leveling-screws  adjusted.  By  placing  the  instruments  on  a  support  having 
a  smooth  and  flat  top,  it  is  evident  that,  barring  accidents,  the  stand  and  stage  will 
remain  indefinitely  accurately  leveled. 

Motion  of  the  Stage.  —  In  order  to  examine  the  entire  surface  of  a  large  specimen 
it  is  necessary  to  bring  in  turn  within  the  field  of  the  microscope  the  different  portions 
of  the  specimen,  and  this  necessitates  the  moving  of  the  stage  in  various  directions. 
The  weight  of  the  stage,  however,  would  create  considerable  friction  between  the  legs 
and  the  supporting  table,  making  the  sliding  motion  jerky  and  otherwise  unsteady. 
To  overcome  this  difficulty  the  back  leg  of  the  stage  is  provided  with  a  small  wheel 
running  in  a  groove  cut  in  a  small  brass  plate  fastened  to  the  table  or  desk,  shown  in 
Figure  13.  The  mounting  of  the  wheel  is  provided  with  a  pivot  fitting  snugly  into  a 


CHAPTER   I  — APPARATUS   FOR  THE   METALLOGRAPHIC   LABORATORY       17 

hole  in  the  leg.  This  construction  makes  possible  the  ready  back-and-forth  motion 
of  the  stage,  as  well  as  its  free  circular  displacement  around  the  axis  of  the  back  leg 
thus  permitting  to  bring  quickly  any  desired  portion  of  the  object  under  the  objec- 
tive. As  the  bulk  of  the  weight  is  supported  by  the  back  leg,  the  arrangement  makes 
possible  a  very  steady  and  smooth  motion  of  the  stage. 

Mechanical  Stage.  —  The  use  of  a  mechanical  stage  is  often  highly  desirable.  This 
is  taken  care  of  in  the  present  instrument  in  two  different  ways:  (1)  by  the  use  of  a 
mechanical  stage  suitably  attached  to  the  electromagnetic  stage^  and  (2)  by  the  use 


Fig.  14.  —  Universal  metalloscope:  electromagnetic  stage  with  mechanical 
stage,  magnetic  specimen  holder,  small  specimen,  and  base-plate. 


of  a  mechanical  stage  independently  mounted  on  a  separate  base  of  the  usual  horseshoe 
pattern. 

The  first  method  is  illustrated  in  Figure  14.  A  mechanical  stage  of  usual  construc- 
tion is  screwed  on  a  brass  plate  provided  with  two  small  pins  fitting  two  correspond- 
ing holes  in  the  magnetic  stage,  thus  securing  a  firm  and  constant  position  for  the 
mechanical  stage.  When  using  a  mechanical  stage,  however,  a  rigid  and  constant 
position  should  also  be  secured  between  it  and  the  microscope  stand.  To  that  effect 
a  brass  plate  is  provided,  with  recesses  to  receive  the  back  legs  of  the  stand  as  well 
as  the  front  legs  of  the  stage,  shown  in  Figure  14.  It  is  then  possible  at  any  time  to 
place  the  microscope  stand  and  the  stage  in  exactly  the  same  relative  positions. 

The  second  method  consists  in  the  use  of  a  mechanical  stage  separately  mounted 
on  an  ordinary  horseshoe  base,  shown  in  Figure  15.  To  secure  a  constant  relative 
position  between  stand  and  stage,  the  foot  of  the  latter  fits  into  recesses  provided  for 
that  purpose  in  the  base-plate. 


18      CHAPTER   I  — APPARATUS   FOR   THE    METALLOGRAPHIC_  LABORATORY 

The  use  of  this  independently  mounted  mechanical  stage  offers  the  additional 
advantage  resulting  from  the  vertical  up-and-down  racking  of  the  stage,  rendering 
unnecessary  any  vertical  adjustment  of  the  light  and  condenser,  as  well  understood 
by  metallographists. 

Examination  of  Transparent  Objects.  —  To  adapt  the  universal  metalloscope  to 
the  examination  of  transparent  objects,  thereby  converting  it  into  an  ordinary  ^micro- 
scope or,  if  desired,  into  a  petrographical  microscope,  a  separate  stage  on  horseshoe 
base  should  be  used,  as  shown  in  Figure  1'5,  when  the  necessary  Abbe  condenser, 


Fig.  15.  — •  Universal  metalloscope:  mechanical  stage  on 
horseshoe  base,  magnetic  specimen  holder,  small  speci- 
men, and  base-plate. 

analyzer,  polarizer,  etc.,  can  readily  be  attached.  The  instrument  is  then  in  no 
way  inferior^to  high-class  microscopes  for  examination  by  transmitted  or  polarized 
light. 


ILLUMINATION  OF  THE  SAMPLES 

Opaque  objects  such  as  metals  and  alloys  must  necessarily  be  examined  by  re- 
flectedjight,  i.e.  by  light  thrown  upon  them  from  above  the  stage,  their  treatment 
differing  in  this  respect  from  that  of  other  microscopic  preparations,  which  are  gen- 
erally examined  by  transmitted  light,  i.e.  by  light  sent  through  them  and  proceed- 
ing from  below  the  stage. 

With  the  low-power  objectives  there  are  two  possible  ways  of  illuminating  opaque 
specimens:  (1)  by  directing  the  light  obliquely  upon  the  object,  and  (2)  by  causing 
the  light  to  fall  perpendicularly  upon  it  by  means  of  so-called  "vertical  illuminators." 


CHAPTER   I  — APPARATUS   FOR  THE   METALLOGRAPHIC   LABORATORY      19 

With  medium-high  and  high-power  objectives  the  second  method  only  is  possible, 
because  the  distance  between  the  specimen  and  the  front  lens  of  the  objective  is  now 
so  small  that  obliquely  reflected  light  cannot  reach  the  surface  under -examination. 
With  very  low-power  objectives  —  i.e.  having  a  focal  length  of  1  inch  or  more  — 
the  vertical  illuminator  may  be  placed  between  the  lens  and  the  object;  but  with 
higher  power  objectives  it  must  of  course  be  inserted  between  the  objective  and  the 
microscope  tube,  the  objective  then  acting  as  a  light  condenser  and  increasing  the 
intensity  of  the  illumination. 

Oblique  illumination  may  be  obtained  (a)  by  allowing  daylight  or  artificial  light 
to  fall  freely  upon  the  object;  (6)  by  directing  the  light  upon  the  object  by  means  of 
mirrors,  reflectors,  or  condensers;  (c)  by  the  use  of  a  "lieberkuhn";  and  (d)  by  the 
use  of  a  "parabolic  reflector." 

Vertical  illumination  may  be  produced  (a)  by  means  of  an  opaque  reflector  con- 
sisting of  a  totally  reflecting  prism  or  of  a  mirror  covering  only  a  portion  of  the  ob- 


(«)  (6) 

Fig.  16.  —  (a)  Oblique  and  vertical  illuminations  of  bright  surface. 
(6)  Oblique  and  vertical  illuminations  of  dull  surface, 
(c)  Oblique  and  vertical  illuminations  of  hills  and  valleys. 


jective,  the  light  returned  by  the  object  reaching  the  eye  by  passing  through  the 
uncovered  portion;  and  (b)  by  means  of  a  transparent  reflector,  generally  a  plain 
glass  disk  or  glass  square,  reflecting  upon  the  object  a  portion  of  the  incident  light 
and  permitting  the  passage  of  a  portion  of  the  light  returned  by  the  object,  which 
thus  reaches  the  eye. 

When  a  highly  polished  surface  is  examined  by  obliquely  reflected  light,  since  the 
angle  of  reflection  is  equal  to  the  angle  of  incidence,  the  totality  of  the  light  is  reflected 
outside  the  objective  (Fig.  16)  and  the  object  appears  uniformly  dark.  In  case  the 
metallic  specimen  contains  some  portions  duller  in  appearance,  these  will  scatter  a 
certain  amount  of  light  a  part  of  which  will  enter  the  objective  (Fig.  16),  and  those 
portions  will  therefore  appear  brighter.  A  similar  effect  is  produced  when  the  speci- 
men, instead  of  being  perfectly  flat,  contains  microscopic  hills  and  valleys,  the  sides 
of  which  may  be  so  inclined  as  to  reflect  some  light  into  the  microscope  (Fig.  16), 
consequently  appearing  bright.  Viewed  by  oblique  light,  therefore,  the  relative  dark- 
ness or  brightness  of  a  constituent  will  vary  inversely  with  its  true  appearance  and 


20      CHAPTER   I  — APPARATUS   FOR   THE   METALLOGRAPHIC   LABORATORY 

will  also  depend  upon  its  orientation,  since  this  will  affect  the  angle  of  incidence  of 
the  light  striking  it.  Generally  speaking,  the  darker  a  constituent  the  brighter  will 
it  seem  to  be  when  illuminated  by  oblique  light,  the  latter  yielding,  so  to  speak,  a  nega- 
tive image.  Oblique  illumination,  moreover,  cannot  be  made  as  intense  as  vertical 
illumination  and,  as  already  explained,  is  possible  only  with  low-power  objectives. 
For  these  and  other  reasons,  while  it  is  not  without  value,  it  is  only  used  occasionally 
by  metallographists. 

To  increase  the  intensity  of  oblique  illumination  and  to  make  its  use  possible  with 
somewhat  higher  powers,  such  appliances  as  the  "  lieberkiihn "  and  the  parabolic  re- 
flector have  been  used.  The  " lieberkiihn,"  so  called  from  the  name  of  its  inventor, 
consists  of  a  small  concave  mirror  attached  to  the  objective  and  reflecting  upon  the 
object  some  light  proceeding  from  below  the  stage  and  passing  around  the  object. 
It  will  be  evident  that  only  small  size  objects  can  be  thus  illuminated. 

The  parabolic  reflector  (Fig.  17),  first  constructed  by  Messrs.  Beck  of  London  for 
Dr.  Sorby,  consists  of  a  parabolic  mirror  placed  on  one  side  between  the  objective 


(a) 

Fig.  17.  —  (a)  Parabolic  reflector. 

(6)  Sorby-Beck  parabolic  reflector. 

and  the  object  and  condensing  the  incident  light  upon  the  latter.  It  should  be  at- 
tached to  the  objective.  Dr.  Sorby  later  added  a  silver  mirror  in  the  shape  of  a  half 
disk  to  the  same  mount,  so  as  to  be  able  to  obtain  at  will  vertical  and  oblique  illumi- 
nation when  using  low-power  objectives  (Fig.  17).  When  vertical  illumination  is 
desired,  the  small  mirror  is  swung  over  the  objective,  covering  only  a  portion  of  it, 
and  directing  vertical  rays  of  light  upon  the  object.  This  combination  is  known  as  the 
Sorby-Beck  reflector. 

The  effects  of  a  vertical  illumination  are  precisely  opposite  to  those  of  an  oblique 
illumination,  as  clearly  shown  in  Figure  16,  highly  polished  surfaces  reflecting  the 
totality  of  the  light  into  the  objective,  while  dull  ones  appear  dull  because  they  reflect 
most  of  the  light  outside. 

To  produce  a  vertical  illumination  we  have  the  choice  between  an  opaque  or  a 
transparent  (glass)  reflector.  The  opaque  reflector  consists  of  a  totally  reflecting 
right-angled  prism,  or  of  a  mirror  placed  between  the  microscope  tube  and  the  objec- 
tive and  covering  only  a  portion  (generally  about  one  half)  of  its  aperture.  The  beam 


CHAPTER   I  — APPARATUS   FOR   THE   METALLOGRAPHIC   LABORATORY      21 


of  light  enters  the  illuminator  through  a  side  opening  provided  for  that  purpose  and 
is  reflected  downwards  by  the  reflector,  being  condensed  upon  the  object  by  the  lenses 
of  the  objective  itself.  The  light  sent  back  by  the  object  reaches  the  eye  through  the 
uncovered  part  of  the  objective. 

The   first  vertical   illuminator  was   designed   by  Prof.  Hamilton  L.  Smith    of 
Hobart  College,   Geneva,  N.  Y.,  and  consisted  of  a  small  annular  silver  mirror 


V 
(W 


(«) 


w 


Fig.  18.  —  (a)  Annular  mirror. 

(6)  Semi-circular  mirror. 

(c)  Central  mirror. 

(d)  Totally  reflecting  prism. 

(e)  Plain  glass  disk. 

(Fig.  18),  forming  an  angle  of  45°  with  the  axis  of  the  microscope,  the  light  reflected 
by  the  object  passing  through  the  central  opening  on  its  way  to  the  eye.  Semi- 
circular mirrors,  similarly  mounted  and  partially  covering  the  objective  (Fig.  18), 
have  been  used  with  equal  satisfaction,  and  the  author  has  obtained  good  results  with 
a  very  small  central  mirror  (Fig.  18)  suitably  mounted,  reflecting  the  light  upon  the 


-K 


A  B 

Fig.  19.  —  Vertical  illuminator.     Totally  reflecting  prism.     (Zciss.) 

central  portion  of  the  objective  lenses,  and  permitting  the  returned  light  to  reach  the 
eye  through  the  free  space  surrounding  the  mirror. 

Instead  of  a  mirror,  a  totally  reflecting  right-angled  prism  may  be  used  as  shown 
in  Figures  18  and  19,  covering  half  of  the  aperture  of  the  objective.  The  prism  is 
so  mounted  that  it  can  be  rotated  around  its  horizontal  axis,  this  being  needed  in  order 
to  secure  the  best  illumination  of  the  sample.  Nachet,  of  Paris,  provides  his  prism 


22      CHAPTER   I  — APPARATUS   FOR   THE   METALLOGRAPHIC   LABORATORY 

with  an  additional  motion  permitting  it  to  cover  a  greater  or  smaller  portion  of  the 
objective.  These  reflecting  prisms  are  now  used  much  more  than  the  reflecting 
mirrors. 

In  1874  Nachet  constructed  for  the  International  Commission  of  the  Meter  some 
objectives  provided  with  totally  reflecting  prisms  as  permanent  parts  of  their  mount- 
ings. In  low-power  objectives  a  prism  was  placed  above  the  first  lens  (Fig.  20),  while 
with  higher  power  objectives  it  was  necessarily  inserted  above  the  double  or  triple 


Fig.  20.  —  Nachet  illuminating  objectives. 

lens  system.  These  objectives  are  called  illuminating  objectives.  This  arrangement, 
however,  has  not  been  found  very  satisfactory  and  with  one  notable  exception  is 
seldom  used  by  metallographists. 

In  vertical  illuminators  having  a  transparent  reflector,  the  latter  consists  of  a  plain 
glass  disk  covering  the  whole  of  the  aperture  of  the  objective  (Figs.  18  and  21).  The 
incident  light  is  in  part  reflected  upon  the  object,  while  another  portion  passes  freely 
through  the  glass  reflector.  A  part  of  the  light  returned  by  the  object  is  again  re- 
flected by  the  glass  illuminator,  while  another  portion  passes  through  it  and  thus 


O4 


Fig.  21.  —  Bausch  and  Lomb  plain  glass  disk  vertical  illuminator. 

reaches  the  eye.  The  glass  reflector  is  so  mounted  that  it  can  be  rotated  around  its 
horizontal  axis  (Fig.  21).  The  amount  of  light  permitted  to  enter  the  illuminator 
may  be  regulated  by  an  iris  diaphragm  attached  to  the  side  opening  or  independently 
mounted  and  placed  between  it  and  the  source  of  light,  or  by  a  revolving  sleeve  at- 
tached to  the  illuminator  and  provided  with  different  size  openings.  The  first  plain 
glass  illuminator  was  constructed  by  Mr.  Beck  of  London. 

With  very  low-power  objectives  it  is  preferable  to  place  the  vertical  illuminator 
between  the  objective  and  the  object,  attaching  it  to  the  former  in  some  suitable  way, 


CHAPTER   I  — APPARATUS   FOR  THE   METALLOGRAPHIC  LABORATORY      23 

as  for  instance  by  providing  the  lower  end  of  the  objective  with  a  society  screw  (see 
Fig.  6). 

While  the  author  is  well  aware  that  some  metallographists  of  note  prefer  the  prism 
to  the  plain  glass  type  of  vertical  illuminator,  in  his  opinion  the  plain  glass  reflector  is 
greatly  superior.  While  the  illumination  obtained  by  its  use  is  not  quite  as  intense, 
it  is  certainly  more  uniform  and  less  liable  to  produce  a  distortion  of  the  image. 

An  improved  construction  of  the  plain  glass  vertical  illuminator  is  illustrated  in 
Figure  21.  The  glass  reflector  is  inserted  into  a  brass  ring  which  on  the  side  opposite 
the  milled  head  is  screwed  into  the  wall  of  the  brass  mounting,  pFactically  doing  away 
with  the  frequent  breaking  of  the  glass  and  greatly  facilitating  its  cleaning.  The  milled 


A- 


— B 


Fig.  22.  —  Vertical  magnifier. 

head  is  large,  which  makes  it  possible  to  impart  a  more  delicate  motion  to  the  glass 
reflector. 

Vertical  Magnifier.  —  For  the  examination  of  specimens  after  the  polishing  opera- 
tion in  order  to  ascertain  the  absence  of  scratches,  for  the  examination  of  fractures, 
etc.,  and  more  especially  for  the  measurement  of  the  diameters  of  the  spherical  de- 
pression of  the  Brinell  test  for  hardness,  the  "vertical"  magnifier,  Figure  22,  has 
been  found  very  useful.  The  plain  glass  reflector  G  placed  at  an  angle  of  45°  causes 
the  specimen  under  examination  to  be  brilliantly  lighted  by  vertical  light  as  in  the 
vertical  illuminator  attached  to  metallurgical  microscopes.  For  the  measurement  of 
small  distances  (as  in  the  Brinell  hardness  test),  a  thin  steel  scale  R,  with  proper 
graduations,  is  inserted  in  a  slot  A  cut  in  the  mounting  and  so  placed  that  the  grad- 
uated edge  meets  the  optical  axis  of  the  lenses. 


SOURCES  OF  LIGHT  AND  CONDENSERS 

The  illumination  of  opaque  objects  such  as  metals  and  alloys  requires  an  intense 
source  of  light,  especially  for  their  photography.  Daylight  and  ordinary  gas  or  oil 
flames  should  be  discarded  as  not  suitable  for  the  purpose,  the  sources  of  light  which 
have  been  found  most  satisfactory  being,  in  the  order  of  their  excellence,  intensity, 
and  decreasing  cost:  (1)  the  electric  arc  lamp,  (2)  the  Nernst  lamp,  and  (3)  the  Wels- 
bach  gas  lamp.  The  author  has  recently  used  with  very  satisfactory  results,  250 
and  500  watt,  nitrogen  filled,  incandescent  tungsten  lamps. 

The  Welsbach  lamp  outfits  (Figs.  23  and  24)  are  very  inexpensive  and  quite  satis- 
factory for  visual  examination  by  low-  and  medium  high-power  objectives.  In  tak- 


24      CHAPTER   I  — APPARATUS   FOR   THE   METALLOGRAPHIC   LABORATORY 

ing  photomicrographs,  however,  their  lack  of  intensity  necessitates  very  long  ex- 
posures, while  with  high-power  objectives  the  light  received  upon  the  camera  screen 
is  so  faint  as  to  render  proper  focusing  of  the  object  a  very  difficult,  if  not  impossible, 
operation. 


Fig.  23.  —  Welsbaeh  lamp  and  double-convex  condensing  lens. 

Two  kinds  of  electric  arc  lamps  are  now  supplied,  one  with  large  carbons  (Fig.  25) 
and  a  smaller  one  with  carbons  measuring  only  }/±  inch  in  diameter  (Fig.  28).  The 
carbons  should  be  placed  at  right  angles,  as  this  arrangement  directs  the  maximum 
amount  of  light  into  the  condensers.  Both  carbons  should  be  cored  and  for  direct 
current  the  vertical  or  negative  carbon  should  be  smaller  than  the  horizontal 


Fig.  24.  — •  Welsbaeh  lamp  and  bull's  eye  condenser. 

carbon.  While  automatic  feeding  of  the  carbons  (Fig.  27)  is  a  valuable  feature,  it 
is  not  by  any  means  essential,  as  remarkably  effective  hand-feed  lamps  are  now 
constructed  by  which  a  very  steady  light  can  be  maintained  (Fig.  26).  Automatic 
mechanisms,  moreover,  are  liable  to  get  out  of  order  and  occasional  sudden  shift- 
ings  of  the  light  are  difficult  to  eliminate  entirely. 


CHAPTER  I— APPARATUS  FOR  THE  METALLOGRAPHIC  LABORATORY   25 


The  large  carbon  lamp  yields,  of  course,  by  far  the  most  intense  illumination  and 
is  the  only  one  suitable  for  direct  projection  of  metallic  samples  upon  a  screen  for 
public  exhibition.  In  taking  photomicrographs  with  the  large  arc  lamp  the  needed 
exposures  are  often  instantaneous  and  seldom  exceed  5  or,  at  the  most,  10  seconds. 
The  lamp  consumes  from  15  to  20  amperes. 


Fig.  2.5.  —  Largo  arc  lamp  outfit. 

The  small  arc  lamp  (Fig.  28)  is  very  satisfactory  for  visual  examination  and  is, 
of  course,  much  less  expensive.  It,  however,  requires  longer  exposures  when  photo- 
graphing. The  position  of  the  carbons  can  be  regulated  with  great  nicety  by  inde- 
pendent adjustments,  thus  securing  a  very  uniform  light.  The  lamp  consumes  about 
5  amperes. 

The  medium  size  arc  lamp  described  on  page  29  as  part  of  a  complete  photomi- 
crographic  outfit  gives  excellent  results  and  is  the  one  preferred  by  the  author. 


Fig.  20.  —  Hand-feed  arc  lamp. 


Fig.  27.  —  Automatic-feed  arc  lamp. 


The  Nernst  lamp  (Fig.  29)  is  used  successfully  by  many  microscopists  and  un- 
doubtedly affords  a  very  satisfactory  illumination  both  for  visual  examination  and 
for  photomicrography.  In  taking  photographs,  exposures  of  10  seconds  or  more  are 
needed,  according  to  the  magnification  and  the  character  of  the  specimen. 

Summing  up,  if  we  desire  a  cheap  and  convenient  form  of  illumination  for  visual 
examination  with  objectives  of  low-  and  medium-high  power,  the  Welsbach  lamp  will 
be  found  in  every  way  satisfactory;  while  for  the  taking  of  photomicrographs  and  for 


26      CHAPTER   I  — APPARATUS   FOR  THE   METALLOGRAPHIC   LABORATORY 

examination  by  high-power  objectives  the  electric  arc  lamp,  the  250  or  more  watt, 
nitrogen  filled,  tungsten  lamp  and  the  Nernst  lamp  should  be  recommended,  bearing 
in  mind  that  the  large  arc  lamp  will  yield  light  of  greatest  intensity  but  will,  on  the 
other  hand,  be  much  more  costly.  When  neither  gas  nor  suitable  electric  current  are 


Fig.  28.  —  Small  electric  arc  lamp,  bull's  eye  condenser,  and  rheostat. 

available,  an  acetylene  lamp  should  be  used,  provided  tanks  of  acetylene  gas  can 
readily  be  obtained. 

Condensers.  - —  Some  kind  of  condensing  attachment  must  be  placed  between  the 
source  of  light  and  the  vertical  illuminator  so  that  a  large  portion  of  the  light  may  be 
utilized  and  a  beam  of  suitable  size  directed  into  the  illuminator.  In  the  case  of  light 
proceeding  from  a  luminous  point  or  at  least  from  a  small  luminous  area,  as  for  in- 


Fig.  29.  —  Nernst  lamp  and  special  bull's  eye  condenser  on  adjustable 

supports. 

stance  with  the  electric  arc,  at  least  two  lenses  or  systems  of  lenses  are  needed,  one 
system,  PL  and  ML  (Fig.  30),  placed  near  the  source  of  light,  to  collect  the  divergent 
rays  and  convert  them  into  a  parallel  beam,  and  a  second  lens  CL  placed  at  some 
distance  from  the  first,  to  convert  the  parallel  beam  into  a  converging  one.  The  ver- . 
tical  illuminator  should  be  located  at  such  a  distance  from  the  condensing  lens  that 
the  beam  of  light  will  cover  a  little  more  than  the  opening  through  which  it  enters 


CHAPTER   I  — APPARATUS   FOR   THE   METALLOGRAPHIC   LABORATORY      27 

the  illuminator.  With  arc  lamps  using  more  than  5  amperes  a  glass  cooling  cell 
CC,  filled  with  distilled  water  or  some  other  suitable  liquid,  should  be  placed  be- 
tween the  two  lenses  in  order  to  absorb  a  large  amount  of  heat  and  thereby  pre- 
vent injury  to  the  objective.  An  iris  diaphragm,  /,  should  also  be  used  to  control 
the  amount  of  light  entering  the  vertical  illuminator.  This  diaphragm  should  be 
placed  in  front  of  the  converging  lens  and  should  be  provided  with  clips  for  holding 
ground  and  colored  glasses.  These  various  parts  should  be  mounted  on  a  so-called 
"optical  bench"  B  upon  which  they  can  slide. 

With  a  large  luminous  body  such  as  the  Welsbach  mantle,  a  single  double-convex 
lens  (Fig.  23)  or  a  bull's  eye  condenser  (planoconvex)  (Fig.  24)  is  sufficient  to  collect 
and  condense  the  necessary  amount  of  light.  It  should,  of  course,  be  placed  at  the 
proper  distance  both  from  the  vertical  illuminator  and  from  the  source  of  light.  The 
use  of  an  iris  diaphragm  attached  to  the  lens  or  on  a  separate  mount  is  advisable, 


CL   CC   PL  ML 


Fig.  30.  —  Condensing  lenses,  cooling  cell,  iris  diaphragm,  automatic  shutter, 

and  optical  bench. 

since  it  affords  a  ready  means  of  controlling  the  amount  of  light  admitted  into  the 
illuminator. 

Monochromatic  Light.  —  The  different  lamps  described  above  all  yield,  of  course, 
white  light,  and  since  the  correction  even  of  apochromatic  objectives  for  chromatic 
aberration  is  never  perfect,  it  is  evident  that  the  use  of  monochromatic  light  —  i.e. 
light  of  one  wave  length  —  is  preferable,  especially  for  photographing.  Monochro- 
matic light  may  be  obtained  in  two  ways:  (a)  by  using  a  source  of  light  actually  mono- 
chromatic, and  (6)  by  causing  white  light  to  pass  through  colored  glass  screens  or 
colored  solutions  (light  filters),  preventing  the  passage  of  some  undesirable  rays.  The 
mercury  arc  lamp  yields  a  nearly  monochromatic  light  and  has  been  tried  by  Le 
Chatelier  with  satisfactory  results.  It  seems  more  convenient,  however,  when  mono- 
chromatic light  is  wanted,  to  use  light  filters  of  suitable  colors,  in  which  case  colored 
glass  screens  will  be  found  easier  to  manipulate  than  glass  cells  containing  colored 
solutions. 

PHOTOMICROGRAPHIC  OUTFITS 

For  taking  photomicrographs,  a  light-tight  connection  should  be  established  be- 
tween the  microscope  and  a  suitable  (Fig.  31)  camera. 

The  Universal  metalloscope  already  described  is  seen  in  Figure  32  with  vertical 
camera  and  Nernst  lamp. 

The  arrangement  the  author  has  found  by  far  the  most  satisfactory  is  illustrated 


28      CHAPTER   I  — APPARATUS   FOR   THE   METALLOGRAPHIC   LABORATORY 


Fig.  31.  —  Photomicrographic  camera  (vertical  position),  showing  metal- 
lurgical microscope,  mechanical  stage,  automatic  shutter,  etc. 


CHAPTER   I  — APPARATUS   FOR   THE    METALLOGRAPHIC   LABORATORY      29 

in  Figure  33.  The  microscope  already  described,  camera,  and  arc  lamp  are  mounted 
on  the  same  rigid  support,  thereby  securing  accurate  alignment  and  ease  of  manipula- 
tions. The  supporting  table  is  of  such  a  height  that  the  observer  may  be  seated  for 
visual  work  and  in  a  comfortable  standing  position  for  the  focusing  of  images  on  the 
screen  of  the  camera  (Fig.  34).  Connection  between  the  microscope  and  camera  is 
very  quickly  and  easily  made.  Plates  measuring  5  by  7  inches  and  smaller  sizes  may 
be  used.  The  source  of  light  is  an  electric  90°  arc  lamp,  supplied  with  a  triple  con- 


Fig.  32.  —  Universal  metalloscope,  Nernst  lamp  outfit,  and  vortical  camera. 

dousing  system  with  lenses  4^  inches  in  diameter.  The  current  may  be  direct  or 
alternating.  The  lamp  is  enclosed  in  a  small  cylindrical  hood  with  observation  win- 
dows. The  carbon  adjustments  are  so  arranged  as  to  be  conveniently  reached  by 
the  observer  either  at  the  microscope  or  at  the  camera  (Fig.  34).  Clips  fastened  to 
the  front  of  the  lamp  permit  the  use  of  monochromatic  glass  screens,  ground  glass, 
etc.  The  light  may  be  tilted  at  any  desired  angle  for  transparent  or  for  oblique 
illumination.  The  electric  arc  lamp  may  be  replaced  with  very  satisfactory  results 
by  a  250  watt,  nitrogen  filled,  tungsten  lamp. 


30      CHAPTER    I  — APPARATUS   FOR   THE   METALLOGRAPHIC   LABORATORY 


Fig.  33.  —  Photomicrographic  apparatus  with  latest  form  of  metallurgical  micro- 
scope (0.1  actual  size). 


CHAPTER   I  — APPARATUS   FOR   THE   METALLOGRAPHIC   LABORATORY      31 


Fig.  34. 


INVERTED  MICROSCOPES 

Le  Chatelier  was  the  first  to  suggest  the  use  of  an  inverted  microscope  for  the  ex- 
amination of  metallic  surfaces.  In  this  style  of  microscope  the  stage  is  placed  hori- 
zontally above  the  objective,  the  latter  being  necessarily  pointed  upwards  (Figs.  35 
to  37). 

In  the  inverted  type  of  microscope  and  photographic  attachment  illustrated  in 
these  pages,  it  has  been  attempted  to  simplify  the  construction  with  corresponding 
material  decrease  in  price. 

The  microscope  is  permanently  connected  with  the  camera  by  a  totally  reflecting 
prism  P  (Fig.  37)  set  rigidly  below  the  vertical  illuminator.  A  separate  tube  set  at 
right  angles  to  the  first  is  provided  for  visual  examination,  another  totally  reflecting 
prism  P'  being  fastened  to  the  inner  end  and  serving  to  reflect  the  image  from  the 
body  tube  through  the  eye  tube  to  the  eye.  When  a  photograph  is  to  be  taken  this 
prism  P'  is  simply  withdrawn  from  the  field  by  means  of  the  draw  tube.  The  eye 
tube  is  fitted  with  pin  and  slot  which  mark  the  limits  to  which  the  small  prism  P' 
may  be  pushed  in  and  withdrawn,  so  that  the  vertical  illuminator  being  once  set,  the 
only  adjustment  necessary  is  at  the  arc  lamp.  With  the  Nernst  and  Welsbach  lamps, 
after  the  light  and  the  vertical  illuminator  are  once  set,  no  more  adjustments  are 
necessary.  The  two  totally  reflecting  prisms  need  never  be  rotated  and  in  fact  can- 
not be  moved,  except  for  the  sliding  motion  of  the  prism  P'  as  already  described. 

The  stage,  which  is  revolving  and  provided  with  centering  screws,  is  of  course 
equipped  with  both  coarse  and  fine  adjustment,  and  a  mechanical  stage  may  readily 
be  substituted  for  the  plain  stage. 

With  this  inverted  microscope  the  use  of  a  magnetic  holder  will  also  be  found  very 
convenient,  for  the  sample,  instead  of  resting  loosely  on  the  stage,  is  then  held  firmly 
in  place  thereby  increasing  the  usefulness  of  the  mechanical  stage. 


32      CHAPTER   I  — APPARATUS   FOR   THK    MKTALLOGRAPHIC   LABORATORY 


Fig.  35.  —  Inverted  metalloscopc  (?,  actual  size). 


CHAPTER   I  — APPARATUS   FOR  THE    METALLOGRAPHIC   LABORATORY      33 

The  placing  of  the  light  on  the  same  side  of  the  microscope  as  the  camera  makes 
it  possible  for  the  operator  to  regulate  his  illumination  while  focusing  the  object  on 
the  camera's  screen. 

One  of  the  valuable  features  of  this  model  is  its  compactness.  While  at  the  camera 
screen,  the  operator  can  easily  reach  the  lamp  adjustments,  and  the  fine  adjustment 


Fig.  36.  —  Inverted  metalloscope  (rear  view). 
Showing  means  of  controlling  fine  adjustment. 


rv 


-t    ± 


Fig.  37.  —  Inverted  metalloscope  (vertical  section,  front  view). 
R  =  Vertical  illuminator  reflector. 

P'  =  Totally  reflecting  prism  which  reflects  image  into  the  eye  tube  when  latter  is  pushed  in. 
P  =  Totally  reflecting  prism  which  reflects  image  into  camera  when  eye  tube  is  pulled  out. 
S  =  Metalloscope  stage. 
M  =  Magnetic  specimen  holder. 
O  =  Specimen. 


of  the  microscope  is  controlled  from  the  same  position  by  means  of  a  thread  belt 
actuated  by  a  small  milled  head  pulley  placed  on  a  standard  at  the  end  of  the  camera 
bed  bar  near  the  screen  (Fig.  36). 


34      CHAPTER   I  — APPARATUS   FOR   THE    METALLOGRAPHIC   LABORATORY 


POLISHING  APPARATUS 

Hand  Polishing.  —  When,  in  spite  of  the  length  and  laboriousness  of  the  opera- 
tion, iron  and  steel  samples  are  to  be  polished  by  hand,  four  smooth  blocks  of  wood 
should  be  prepared,  some  6  by  10  inches  and  1  inch  thick.  Two  of  these  should  bo 
covered  with  canvas  or  linen  duck  and  the  others  with  fine  broadcloth.  The  blocks 
are  to  be  used  as  described  in  Chapter  II. 

Polishing  by  Power.  —  The  power  polishing  machine  shown  in  Figure  38  has  been 
found  very  satisfactory.  It  consists  of  a  heavy  iron  pedestal  upon  which  is  mounted 


Fig.  38.  —  Power  polishing  machine. 

a  grinder  having  emery-wheel  and  cast-iron  disks  revolving  in  a  vertical  piano,  thus 
giving  four  polishing  surfaces  of  graduated  fineness.  The  polishing  powders  mixed 
with  water  are  applied  to  the  various  disks  by  means  of  brushes,  and  shields  are  pro- 
vided to  catch  any  surplus  water  that  may  be  thrown  off  during  the  operation.  Should 
a  cloth  become  worn  or  torn  it  is  readily  and  quickly  replaced.  This  machine  very 
much  shortens  the  time  necessary  for  the  preparation  of  samples  and  is  far  superior 
to  those  where  only  one  block  is  made  to  rotate  at  a  time.  A  speed  of  1200  revolu- 
tions per  minute  has  been  found  most  satisfactory  for  polishing  iron  and  steel  samples, 


CHAPTER   I  — APPARATUS   FOR   THE    METALLOGRAPHIC   LABORATORY      35 

but  by  the  use  of  a  variable  speed  electric  motor  to  run  the  polishing  machine  various 
speeds  may  be  readily  obtained. 

The  polishing  motor  shown  in  Figure  39  possesses  the  advantage- of  directly  driven 
over  belt  driven  machinery.  It  is  provided  with  the  same  polishing  disks  as  the  pol- 
ishing machine  and  can  be  built  both  for  constant  and  for  variable  speed. 

The  operation  of  polishing  with  these  machines  is  described  in  Chapter  II. 


Fijj.  39.  —  Polishing  motor. 


Fig.  40.  —  Portable  polishing  motor. 


Portable  Polishiixj  Motor.  —  A  portable  polishing  motor  is  shown  in  Figure  40. 
It  has  been  devised  for  the  purpose  of  polishing  small  spots  (about  %  inch  in  diameter) 
on  pieces  too  large  to  be  treated  in  the  ordinary  way  and  from  which  samples  of  suit- 
able dimensions  cannot  readily  be  cut,  as  for  instance  finished  forgings  or  castings, 
etc.  It  consists  of  a  ^g  H.  P.  motor,  flexible  shaft,  grinding  heads,  and  polishing  disks 
suitably  covered.  A  spot  may  be  satisfactorily  polished  in  some  15  to  20  minutes. 


36     CHAPTER  I  — APPARATUS  FOR  THE  METALLOGRAPHIC  LABORATORY 

PYROMETERS  AND  ELECTRIC  FURNACES 

Pyrometers.  —  The  Le  Chatelier  thermo-electric  pyrometer  is  undoubtedly  the 
instrument  best  adapted  to  the  measurement  of  temperatures  needed  to  control  such 
heat  treatments  as  are  likely  to  be  performed  in  a  metallographical  laboratory.  The 


Fig.  41.  —  Siemens  and  Halske  galvanometer. 


Fig.  42.  —  Leeds  and  Northrup  thermo-couple  potentiometer. 

thermo-couple  consists  of  a  wire  of  pure  platinum  and  of  a  wire  of  platinum  alloyed 
with  10  per  cent  of  rhodium  or  iridium.  To  measure  the  electromotive  force  created 
an  accurate  direct  reading  galvanometer  should  be  used  (Fig.  41)  or  else  the  poten- 


CHAPTER  I  — APPARATUS  FOR  THE   METALLOGRAPHIC  LABORATORY      37 

tiometer  method  (Fig.  42).  Instructions  for  the  use  of  these  instruments  are  generally 
supplied  by  the  makers. 

The  use  of  cheaper  couples  and  cheaper  instruments  is  not  to  be  commended,  for 
they  are  unsuitable  for  accurate  scientific  work  especially  at  high  temperatures. 

An  autographic  recording  pyrometer  is  very  useful  and  quite  indispensable  for 
the  detection  of  faint  evolutions  or  absorptions  of  heat.  Indeed  without  its  use  there 
are  many  delicate  thermal  treatments  that  could  not  be  performed.  Several  auto- 
graphic instruments  are  now  constructed.  To  meet  the  needs  ofj.he  metallographist 


Pt.—  Rli. 
Fig.  43.  —  Saladin  self-recording  thermo-electric  pyrometer. 


the  author  believes  that  the  pyrometric  recorder  designed  by  Le  Chatelier  and  Sala- 
din and  constructed  by  Pellin  of  Paris  (Figs.  43  to  45)  will  be  found  most  satisfactory. 
In  an  early  form  the  different  parts  were  arranged  as  shown  in  Figure  43.  The  light 
proceeding  from  the  source  S  after  passing  through  a  lens  is  received  by  the  mirror 
of  a  sensitive  galvanometer  GI  the  deflections  of  whicli  measure  the  difference  in 
temperature  between  the  sample  under  examination  and  the  neutral  body.  This 
horizontal  deflection  of  the  beam  of  light  is  converted  into  a  vertical  deflection  by 
passing  through  a  totally  reflecting  prism  M  placed  at  an  angle  of  45  degrees. 

This  vertically  moving  beam  of  light  is  received  by  the  mirror  of  a  second  gal- 
vanometer (?2  whose  deflections  are  a  measure  of  the  temperature  of  the  sample.    The 


38      CHAPTER   I  — APPARATUS   FOR   THE    METALLOGRAPHIC    LABORATORY 

beam  then  passes  through  a  lens  and  reaches  the  screen  or  plate  P.  L  is  a  lens  at  the 
conjugate  foci  of  which  are  placed  the  mirrors  of  the  two  galvanometers.  Two 
motions  are  in  this  way  imparted  to  the  spot  of  light,  (1)  a  horizontal  motion  pro- 
portional to  the  temperature  of  the  sample  and  (2)  a  vertical  motion  proportional  to 


Fig.  44.  —  Le  Chiitclicr-Saladin  self-recording  thermo-electric  pyrometer. 


A  For  ordutary 
Cooliny  Curves 


IVlOftlr. 


p. 

( 


Sensitvfe 

Gei£vuns>mcfar 


Vacuous  Porcetuifi.  Z'ujte 


Fig.  4.5.  —  Roberts-Austen  method  of  connecting  the  sample,  neutral  body, 
and  galvanometers. 


the  difference  in  temperature  between  the  sample  and  the  neutral  body.  The  result- 
in»-  curves  are  known  as  differential  curves  (see  Chapter  X). 

In  recent  models  the  apparatus  has  been  simplified  and  made  more  compact 
(Fig.  44)  by  placing  the  galvanometers  so  near  each  other  that  the  lens  L,  Figure  43, 
could  be  omitted  and  the  entire  instrument  placed  in  a  metallic  or  wooden  case. 

The  apparatus  is  mounted  on  an  aluminium  base  B  provided  with  three  leveling- 
screws  VV'V"  and  a  level  N.  The  optical  and  electrical  parts  are  enclosed  in  a  ver- 


CHAPTER   I  — APPARATUS   FOR  THE   METALLOGRAPHIC   LABORATORY      39 

tical  aluminum  box.  The  suspensions  of  the  galvanometers  are  regulated  through 
the  milled  head  bbi  attached  to  the  suspension  wires.  The  light  proceeding  from  the 
source  at  L  (Nernst  or  incandescent  lamp)  fall  on  the  collimator  0  provided  with  a 
diaphragm.  D  is  the  camera  and  R  the  screen.  A  small  collimator  C  lighted  by  the 
source  L  throws  its  image  on  a  small  auxiliary  mirror  which  projects  through  the 
slit  F  a  luminous  spot  on  the  scale  E,  making  it  possible  to  follow  from  the  outside 
the  traveling  of  the  luminous  spot  on  the  photographic  plate  during  exposures. 

The  connections  between  the  sample  and  the  neutral  body_and  between  these 
and  the  galvanometers  are  made,  as  first  suggested  by  Roberts-Austen,  and  as  clearly 
shown  in  Figure  45.  In  this  illustration  galvanometer  Gi  corresponds  to  galvanom- 
eter (?2  of  Figure  43  and  galvanometer  G2  to  galvanometer  Gi. 


ELECTRIC   FURNACES 

For  the  experimental  treatment  of  small  samples  of  iron  and  steel  an  electric  re- 
sistance furnace  is  extremely  useful.  Satisfactory  types  of  platinum  wound  furnaces 
are  supplied  by  makers.  For  temperatures  not  exceeding  1000  deg.  C.  furnaces 
wound  with  nichrome  wire  can  be  readily  and  cheaply  built  in  any  laboratory,  both 
muffle  and  tube  pattern.  For  very  high  temperatures  (1200  to  1600  deg.)  kryptol 
furnaces  are  to  be  recommended.  When  iron  and  steel  samples  are  to  be  heated  in 
vacuum,  under  pressure  or  in  an  atmosphere  of  various  gases,  special  furnaces  must 
be  built  provided  with  vacuum  apparatus,  pressure  pumps,  gauges,  etc. 


CHAPTER  II 

MANIPULATIONS 

The  author's  manipulations  are  described  in  this  chapter,  at  some  length;  others 
more  briefly  in  Chapter  III. 

Cutting  of  Samples.  • —  Samples  of  iron  and  steel  for  microscopical  examination 
should  not  generally  exceed  %  inch  square  or  round.  If  the  metal  is  sufficiently  soft 
they  can  be  cut  readily  with  a  hand  saw  or,  preferably,  with  a  power  hack  saw.  If 
too  hard  emery  disks  or  diamond  saws  should  be  used.  Suitable  pieces  of  hard, 
brittle  metals  (white  cast  iron,  spiegeleisen,  etc.)  may  sometimes  be  obtained  by 
breaking  with  a  hammer. 

Polishing  by  Hand.  —  The  outfit  described  in  Chapter  I  should  be  used.  The 
sharp  edges  of  the  sample  should  be  filed  or  ground  in  order  to  avoid  tearing  the 
polishing  cloths  in  the  following  operations.  The  surface  to  be  prepared  should  be 
filed  first  with  a  coarse  and  then  with  a  smooth  file  so  as  to  obtain  a  perfectly  flat, 
smooth  surface.  This  filing  can  be  advantageously  replaced  by  grinding  on  a  fine 
emery-wheel.  It  is  recommended  that  both  filing  or  grinding  be  conducted  with  a 
very  gentle  pressure. 

A  small  amount  of  No.  80  emery-powder1  mixed  with  sufficient  water  to  form  a 
thick  paste  should  be  placed  on  one  of  the  polishing  blocks  covered  with  linen  cloth. 
This  paste  should  be  spread  over  the  block,  conveniently  by  means  of  a  spatula, 
and  with  the  addition  of  a  little  more  water  if  necessary.  The  sample  of  metal  should 
now  be  rubbed  back  and  forth  over  this  block,  being  careful  to  rub  always  in  the 
same  direction  until  the  marks  left  by  the  file  or  emery-wheel  have  all  been  removed 
and  replaced  by  finer  markings  due  to  the  action  of  the  emery-powder.  After  this 
treatment  the  sample  should  be  carefully  washed,  as  well  as  the  fingers  of  the  operator, 
preferably  in  running  water,  and  the  sample  rubbed  over  the  second  polishing  block 
covered  with  linen  cloth  and  a  little  flour-emery  and  water,  precisely  as  before.  On 
passing  from  the  first  to  the  second  polishing  block,  the  sample  should  be  turned  at 
right  angles  and  kept  in  that  position,  in  order  that  the  new  marks  may  be  perpen- 
dicular to  the  old  ones,  and  the  polishing  should  be  continued  until  the  marks  left 
by  the  coarse  emery  have  been  entirely  effaced  and  replaced  by  finer  ones.  The 
sample  after  being  carefully  washed  is  ready  for  the  next  block.  Some  of  the  tripoli 
powder  should  be  spread,  with  the  addition  of  water,  over  one  of  the  blocks  covered 
with  broadcloth,  and  the  sample  polished  upon  this  block  until  the  markings  left 
by  the  previous  polishing  have  been  completely  removed.  After  careful,  washing 
the  sample  should  now  be  rubbed  over  the  last  polishing  block,  covered  with  broad- 
cloth, rouge,2  and  water,  holding  as  usual  the  sample  so  as  to  rub  it  at  right  angles  to 
the  markings  left  by  the  tripoli.  After  these  markings  have  been  removed  the  sample 
should  have  a  very  bright  surface  and  be  free  from  even  microscopical  scratches. 

1  The  polishing  powders  should  be  of  the  very  best  quality  obtainable. 

2  It  is  essential  to  use  the  best  commercial  grade  of  jeweler's  rouge. 

40 


CHAPTER   II  —  MANIPULATIONS  41 

At  this  stage  a  magnifying-glass  is  very  useful  for  inspecting  the  specimen  in  order  to 
ascertain  whether  it  is  ready  for  the  etching  treatment.  For  this  purpose  the  vertical 
magnifier  described  and  illustrated  in  Chapter  I  will  be  found  very  satisfactory. 

The  specimen  should  now  be  carefully  washed  and  dried  with  a  soft  cloth,  pref- 
erably a  fine  piece  of  old  linen.  Where  an  air  blast  is  at  hand,  as  is  generally  the 
case  in  chemical  laboratories,  it  is  advisable  to  dry  the  specimen  with  this  blast  (a 
hot  blast  is  more  effective  than  a  cold  one)  instead  of  rubbing  it  with  a  cloth.  The 
sample  may  then  be  passed  gently  once  or  twice  on  a  piece  _of_chamois  leather 
stretched  over  a  smooth  piece  of  wood  and  carefully  protected  from  dust  when  not  in 
use,  when  it  will  be  ready  for  the  next  or  etching  operation.  When  polishing,  the 
sample  should  be  pressed  lightly  upon  the  blocks  and  great  care  taken  not  to  carry 
any  coarse  powder  over  a  polishing  block  upon  which  a  finer  powder  is  used  as  the 
presence  of  but  a  few  coarser  grains  will  greatly  lengthen  the  operation.  It  is  of 
much  importance,  therefore,  to  keep  all  the  blocks  carefully  covered  when  not  in 
use  as  well  as  the  bottles  containing  the  powders. 

Polishing  by  Power.  —  Polishing  by  hand  is  at  best  a  tedious  and  laborious 
operation  and  whenever  possible  it  is  highly  advisable  to  replace  it  by  the  use  of  a 
power  polishing  machine.  Very  satisfactory  and  effective  polishing  machines  and 
polishing  motors  have  been  described  in  Chapter  I. 

When  using  these  polishing  machines  or  polishing  motors  the  manipulations  are 
as  follows: 

The  metal  surface  to  be  prepared  is  pressed  lightly  upon  the  emery-wheel  until  a 
perfectly  flat  surface  is  obtained,  when  it  should  be  washed  with  the  usual  precau- 
tions and  pressed  upon  the  cloth-covered  cast-iron  disk  placed  next  to  the  emery- 
wheel  and  upon  which  flour-emery  and  water  have  been  applied.  Care  should  be 
taken  to  hold  the  specimen  so  that  the  new  marks  will  cross  the  old  ones  at  right 
angles  and  the  grinding  should  be  continued  until  the  emery-wheel  marks  have  been 
completely  erased.  After  washing  the  specimen  it  is  ready  for  treatment  on  the 
next  surface  covered  with  broadcloth  upon  which  has  been  spread  tripoli  powder 
and  water,  here  again  turning  the  sample  90  degrees.  When  the  marks  left  by  the 
preceding  operation  have  been  removed,  the  specimen  is  washed  and  given  the  final 
polishing  treatment  by  pressing  it  lightly  upon  the  other  side  of  the  cast-iron  disk 
upon  which  rouge  and  water  is  used.  The  various  polishing  powders  mixed  with 
water  may  be  conveniently  applied  to  their  respective  disks  by  means  of  flat  and 
rather  stiff  brushes.  The  surface  of  a  properly  polished  sample  should  be  highly 
specular  and  free  from  scratches. 

Time  will  be  saved  by  exerting  a  slight  pressure  only  while  polishing,  especially 
on  the  emery-wheel  and  emery-disk,  because  deep  marks  due  to  these  abrasers  will 
be  troublesome  to  remove  with  the  finer  powders.  With  these  machines  a  sample  of 
steel  measuring  Yz  inch  square  or  Yi  inch  in  diameter  is  readily  polished  in  10  minutes. 

Polishing  very  Small  Specimens.  —  For  the  polishing  of  samples  too  small  to  be 
conveniently  held  in  the  hand  and  also  for  preventing  the  rounding  off  of  the  edges 
the  author  following  Boylston's  suggestions  uses  small  iron  caps,  known  in  the  trade 
as  "malleable  gas  caps,"  in  two  sizes,  namely,  %  inch  and  Y±  inches.  --  They  cost 
three  to  four  cents  each. 

The  cap  used  as  a  crucible  is  heated  to  about  200  deg.  C.  by  placing  it  on  an  iron 
plate  heated  by  a  bunsen  burner.  It  is  then  filled  with  a  fusible  alloy  recommended 
by  Campion  and  Ferguson  and  consisting  of  50  parts  bismuth,  30  parts  lead,  25 


42  CHAPTER   II  —  MANIPULATIONS 

parts  tin,  and  3  parts  zinc.  This  alloy  melts  below  100  deg.  C.  and  if  used  in  stick 
form,  the  heated  iron  cap  can  readily  be  filled  with  molten  alloy.  The  temperature 
of  the  cap  and  its  contents  is  kept  a  little  above  100  deg.  and  the  specimen  to  be 
polished  pressed  into  it.  After  solidification  and  cooling,  which  can  be  hastened  by 
dipping  in  water,  the  whole  is  polished  in  the  usual  way. 

By  using  iron  caps  instead  of  brass  cylinders  or  other  non-magnetic  mounts  the 
magnetic  holders  described  in  Chapter  I  can  be  used  with  great  gain  in  simplicity 
and  convenience. 

Etching.  —  If  the  polished  samples  were  now  placed  under  the  microscope  it 
would  be  possible  to  detect  the  presence  of  inclusions  such  as  graphite  particles,  slag, 
etc.,  and  if  a  relief  effect  has  been  produced,  the  presence  of  constituents  considerably 
harder  than  the  matrix  (free  cementite  in  hyper-eutectoid  steel,  for  instance)  but 
the  structure  of  the  metallic  matrix  itself  would  not  be  revealed,  because  its  con- 
stituents being  equally  bright  and  exactly  in  the  same  plane  would  reflect  the  light 
to  the  same  extent,  after  the  fashion  of  a  mirror.  To  make  the  structure  apparent 
under  the  microscope  it  is  necessary  to  impart  unlike  appearances  to  the  constituents. 
This  is  generally  accomplished  by  producing  a  slight  corrosion  or  etching  of  the  pol- 
ished surface.  For  this  purpose  acid  solutions  are  generally  used  which  attack  some 
constituents  more  deeply  than  others  or  to  the  exclusion  of  others,  an  action  which 
may  or  may  not  be  accompanied  by  the  deposition  of  some  precipitated  matter. 

Arnold  considers  the  operation  of  etching  with  dilute  acids  to  be  of  an  electro- 
lytic nature,  some  of  the  constituents  being  electro-negative  to  others,  hence  the 
attack  of  some  of  these  (electro-positive  constituents)  to  the  exclusion  of  others 
(electro-negative  constituents)  and  the  darker  coloration  of  the  former. 

For  the  development  of  the  microstructure  of  polished  samples  of  iron  and  steel 
in  general,  the  author's  preference  is  as  follows:  (1)  solution  of  nitric  acid  in  alcohol, 
(2)  solution  of  picric  acid  in  alcohol,  and  (3)  concentrated  nitric  acid. 

Nitric  Acid  and  Alcohol.  —  A  solution  should  be  prepared  containing  10  per  cent 
of  concentrated,  chemically  pure  nitric  acid  and  90  per  cent  of  absolute  alcohol.  A 
small  amount  of  this  solution  should  be  poured  in  a  glass  or  porcelain  dish,  preferably 
a  small  crystallizing  glass  dish  with  cover,  and  the  sample  immersed  in  it  for  10 
seconds,  when  it  should  be  removed,  conveniently  with  a  pair  of  pincers  (preferably 
with  platinum  tips),  and  washed  in  alcohol.  The  sample  should  now  be  dried,  prefera- 
bly by  means  of  a  blast  for  which  a  foot^blower  will  answer  very  well.  After  rubbing 
the  sample  very  gently  once  or  twice  upon  a  smooth  piece  of  chamois  leather  stretched 
on  a  wooden  block  and  carefully  kept  free  from  dust,  it  will  be  ready  for  examina- 
tion. In  case  the  etching  is  not  sufficiently  deep  the  treatment  should  be  repeated. 

Picric  Acid.  (Igevsky.)  —  An  etching  solution  should  be  prepared  containing 
5  grams  of  picric  acid,  chemically  pure,  and  95  cubic  centimeters  of  absolute  alcohol. 
This  should  be  kept  in  a  well-stopped  glass  bottle.  The  samples  should  be  immersed 
in  it  for  30  seconds,  washed  in  alcohol,  dried,  and  the  treatment  repeated  if  necessary. 

Concentrated  Nitric  Acid.  (Sauveur.) — The  polished  specimen  conveniently 
held  with  a  pair  of  pincers  (preferably  with  platinum  tips)  should  lie  dipped  in  a 
beaker  or  other  vessel  containing  concentrated  nitric  acid  (1.42  specific  gravity) 
and  immediately  afterwards  held  under  an  abundant  stream  of  running  water.  When 
iron  is  immersed  in  concentrated  nitric  acid  it  assumes  the  passive  state,  that  is  it  is 
not  affected  by  it.  As  soon,  however,  as  the  layer  of  concentrated  acid  which  covers 
the  polished  surface  is  diluted  by  the  running  water,  the  steel  is  vigorously  attacked  but 


CHAPTER   II  —  MANIPULATIONS  43 

for  so  short  a  time  (since  the  water  soon  removes  all  traces  of  acid)  that  there  is  little 
danger  of  etching  too  deeply.  One  such  treatment  is  generally  sufficient  to  bring  out 
the  structure  sharply  and  clearly  but  if  the  specimen  is  found  insufficiently  etched, 
the  etching  should  be  repeated  in  exactly  the  same  manner.  The  author  believes 
that  the  simplicity  of  this  etching  treatment  and  the  excellent  results  generally  ob- 
tained have  been  overlooked  by  metallographists. 

Sodium- Picrate  Etching  of  Cementite.  —  Cementite  is  not  acted  upon  by  the  usual 
reagents  employed  for  the  etching  of  steel  sections  but  remains_brilliant  and  struc- 
tureless. Kourbatoff,  however,  discovered  a  reagent  which  deeply  colors  cementite 
while  leaving  the  ferrite  unaffected,  thus  affording  a  sure  means  of  distinguishing 
between  the  two.  The  treatment  consists  in  immersing  the  polished  sample  in  a 
boiling  solution  of  sodium  picrate  in  an  excess  of  sodium  hydroxide  for  some  5  to  10 
minutes,  when  the  cementite  assumes  a  brown  to  blackish  coloration.  The  etching 
solution  may  be  prepared  by  adding  2  parts  of  picric  acid  to  98  parts  of  a  solution 
containing  25  per  cent  of  caustic  soda,  for  instance  2  grams  of  picric  acid  in  98  cubic 
centimeters  of  a  solution  made  up  of  24.5  grams  of  caustic  soda  and  73.5  cubic  centi- 
meters of  water. 

Stead's  Reagent  for  the  Detection  of  Phosphorus  Segregation  in  Iron  and  Steel.  — •  For 
the  detection  of  phosphorus  segregation  in  iron  and  steel  Stead  recommends  the  fol- 
lowing method  which  he  considers  superior  to  his  heat-tinting  method: 

The  reagent  is  made  by  mixing  — 

Cupric  chloride    10  grams. 

Magnesium  chloride  ....  40 

Hydrochloric  acid  20  cubic  centimeters. 

Alcohol  to  make  up  to  .  .  1000  " 

The  salts  are  dissolved  in  hot  water  to  saturation  and  the  solution  made  up  to 
1000  cubic  centimeters  with  alcohol.  Magnesium  chloride  is  not  necessary,  but  better 
results  are,  if  anything,  obtained  by  having  it  present. 

"The  specimens  are  simply  covered  with  a  thin  layer  of  the  reagent  and  must  on 
no  account  be  immersed  in  a  bath  of  the  liquid.  The  layer  of  liquid,  after  remaining 
on  the  surface  for  one  minute,  is  shaken  off  and  a  second  layer  dropped  on  the  surface 
and  left  there  for  the  same  period.  This  procedure  is  repeated  as  often  as  it  is  found 
desirable.  The  specimen  is  washed  with  boiling  water,  then  with  methylated  spirits, 
and  shaken  to  remove  the  spirit  from  the  surface,  the  heat  imparted  by  the  boiling 
water  rapidly  evaporating  the  last  traces  of  alcohol.  The  copper  is  usually  left  as  a 
hard  coherent  layer,  and  is  not  readily  removed  by  rubbing;  indeed,  the  specimens 
may  sometimes  be  repolished  by  rouged  chamois  leather  without  disturbing  the  de- 
posited copper.  The  reagent  not  only  indicates  variations  of  phosphorus,  but  by 
progressive  etching  one  can  get  a  very  good  idea  as  to  the  degree  of  difference  between 
the  phosphorus  in  different  portions  of  the  same  metal.  If  the  reagent  be  applied  in 
successive  portions  and  the  difference  in  the  proportion  of  phosphorus  be  slight,  as 
stated  above,  the  copper  invariably  precipitates  on  the  purer  portion  first;  but  on  re- 
peated application  the  copper  gradually  deposits  also  on  the  parts  richer  in  phosphorus, 
and  after  many  applications  of  the  reagent  the  whole  surface,  including  the  phos- 
phorised  parts,  becomes  coated  with  copper.  If,  however,  the  phosphorus  be  very 
much  concentrated  in  one  or  more  parts,  these  remain  perfectly  bright  and  free  from 
copper  even  after  ten  applications  of  the  reagent." 


44  CHAPTER   II  —  MANIPULATIONS 

Heat-Tinting.  —  Heat-tinting  is  described  by  Stead,  its  originator,  as  follows: 
"Heat-tinting  consists  in  heating  polished  specimens  of  metals  until  their  surfaces 
become  colored  by  oxidation  films. 

"Alloys  of  iron  and  phosphorus,  and  commercial  steel,  contain  part  of  their  mass 
richer  in  phosphorus  than  other  portions.  In  fact  the  iron  and  the  phosphide  are 
seldom  intimately  mixed  in  ordinary  steel.  When  polished  specimens  are  placed  on 
the  surface  of  a  molten  bath  of  tinman's  solder,1  and  the  heat  gradually  raised,  the 
portions  of  the  specimens  richest  in  phosphorus  assume  oxidation  tints  earlier  than  the 
purer  parts;  hence  it  follows  that  by  the  time  the  phosphorus-rich  parts  have  passed 
through  the  transition  stages  of  yellow-brown,  to  red  and  purple,  the  purer  portions 
will  have  reached  the  yellow-brown  or  red  stage,  and  if  at  this  point  the  specimen  be 
removed  from  the  source  of  heat,  the  phosphorus-rich  portions  will  appear  under  the 
microscope  as  purple  or  blue  on  a  yellow-brown  or  red  background.  If  the  heating 
of  the  specimen  be  continued,  the  phosphorised  part  will  assume  a  yellowish-white 
tint,  while  the  purer  part  will  reach  the  blue  stage.  Each  part  will  pass  through  the 
complete  range  of  color  from  yellow  to  blue  and  then  to  nearly  white,  but  not  at  the 
same  time,  because  the  purer  portions  always  lag  behind,  the  degree  of  lag  depending 
on  the  variation  in  the  proportions  of  phosphorus  in  the  different  parts. 

Heat-tinting  is  also  useful  in  intensifying  the  difference  in  color  between  ferrous 
sulphide  and  manganese  sulphide  when  present  together  in  steel.  On  heating  polished 
metal  containing  inclusions  of  each  sulphide  until  it  appears  to  assume  a  uniform 
brown  tint,  the  ferrous  sulphide,  which  is  naturally  pale  yellow,  will  be  found  under 
the  microscope  to  have  been  colored  purple,  while  the  manganese  sulphide,  naturally 
a  pale  dove  color,  will  have  become  white.  If  the  heating  be  continued  until  the  sur- 
rounding metal  becomes  blue,  the  ferrous  sulphide  will  be  blue  and  the  manganese 
sulphide  yellow. 

"To  obtain  good  results  by  heat-tinting,  it  is  absolutely  necessary  first  to  apply  to 
the  surface  a  very  dilute  solution  of  some  acid  in  alcohol.  Picric  acid  answers  admi- 
rably, but  care  must  be  taken  to  remove  the  solution  employed  before  it  has  time  to 
develop  the  pearlite  or  sensibly  to  etch  the  metal.  After  thoroughly  washing  the 
specimen  in  water,  it  is  dried  with  a  perfectly  clean  rag  and  heated  on  a  hot  plate  to 
about  150  deg.  C.  It  is  again  rubbed  with  a  warm  clean  cloth,  and  is  then  ready  for 
heating  to  produce  the  color  tint. 

"It  is  difficult  to  explain  why  the  preliminary  acid  treatment  is  necessary,  but  that 
it  is  so  is  proved  in  practice,  for  if  it  is  omitted,  the  tinting  is  always  unsatisfactory. 
It  is  possible  that,  during  polishing,  some  of  the  softer  metal  becomes  spread  over 
the  harder  part,  forming  an  exceedingly  thin  layer.  This,  however,  is  only  a  surmise." 
Sulphur  Printing.  —  The  presence  of  sulphur  in  steel,  especially  when  segregated, 
can  be  sometimes  clearly  revealed  by  a  laboratory  test  known  as  "sulphur  printing." 
As  first  described  by  Heyn  and  Bauer,  it  consisted  in  pressing  upon  the  previously 
polished  steel  surface  strips  of  silk  impregnated  with  mercuric  chloride  and  hydro- 
chloride  acid  (10  grams  mercuric  chloride,  20  cubic  centimeters  of  water,  and  100 
cubic  centimeters  of  hydrochloric  acid).  The  reaction  between  the  acid  and  the  sul- 
phides wherever  present  generates  H2S  (sulphuretted  hydrogen)  which  in  turn  reacts 
with  the  mercury  salt  producing  stains  of  mercury  sulphide  varying  in  intensity 
according  to  the  amount  of  sulphur  present  in  the  steel  and  revealing  the  spots  where 
the  latter  occurs. 

1  An  iron  plate  heated  by  a  bunsen  gas  burner  may  be  used  instead  (Author). 


CHAPTER   II  —  MANIPULATIONS  45 

A  better  method  (Baumann)  consists  in  substituting  for  the  silk  ordinary  silver 
bromide  (photographic)  paper  moistened  with  dilute  sulphuric  acid,  the  generated 
H2S  producing  dark  stains  on  the  paper  where  metallic  sulphides  were  present  in 
the  steel. 

The  following  more  explicit  instructions  may  be  followed  with  good  results. 
The  sample  to  be  tested  should  be  filed  flat  and  rubbed  on  two  or  three  grades  of 
emery-paper,  finishing  with  French  paper  No.  0.  Sheets  of  Velox  printing  paper  or 
its  equivalent  should  be  soaked  until  saturated  in  dilute  sulphuric  acid  (2  per  cent 
solution).  The  paper  should  then  be  placed  on  a  piece  of  plate"  glass  and  the  steel 
specimen  after  washing  it  in  clear  water,  gently  pressed  upon  it  for  some  20  seconds, 
when  it  should  be  removed  and  the  paper  placed  in  sodium  hyposulphite  to  remove 
the  excess  bromide,  washed  and  dried.  While  theoretically  areas  containing  phos- 
phorus should  also  darken  the  bromide  paper  Stead  reports  that  the  staining  from 
this  cause  is  almost  imperceptible. 

Law  covers  the  steel  section  with  a  coating  of  gelatine  containing  an  acid  solution 
of  lead,  mercury,  or  cadmium  salt;  the  acid  decomposes  the  iron  or  manganese  sul- 
phides and  the  resulting  H2S  reacting  with  the  lead  or  cadmium  salt  produces  brown 
or  yellow  stains  of  lead,  mercury,  or  cadmium  sulphide.  Rohl  writes:  "Care  should 
be  taken  that  the  gelatine  be  as  thick  as  possible,  so  as  to  be  stiff  enough  to  avoid  its 
flowing  off  or  shifting  its  position  after  being  applied.  The  gelatine  should  be  applied 
hot  to  the  slide  by  means  of  a  glass  rod.  This  method  permits  of  testing  under  the 
microscope,  for  its  sulphidic  nature,  an  enclosure  placed  in  the  microscope  and  ob- 
served thereon,  the  stage  being  screwed  back  for  the  purpose  of  applying  the  gelatine, 
so  that  the  place  observed  by  the  eye  may  be  at  once  observed  again  in  the  micro- 
scope. Weak  magnification  has  been  found  sufficient  and  practical  for  this  purpose." 

F.  Rogers  describes  a  method  for  taking  sulphur  prints  of  fractures,  consisting  in  the 
use  of  a  gelatine  emulsion  of  silver  bromide  coated  upon  a  very  stiff  grease  clay  and 
soaked  in  a  dilute  acid  solution  containing  also  a  toughening  agent.  This  emulsion 
is  immediately  pressed  into  contact  with  the  clear  fracture  for  a  few  seconds  and 
withdrawn. 

According  to  Rohl,  ferrous  sulphide  is  considerably  darkened  by  one  per  cent  so- 
lution of  organic  acids  in  ethyl  alcohol,  after  5  minutes'  etching,  as  compared  with 
manganese  sulphide.  The  same  author  writes  that  after  a  short  preliminary  etching 
with  alcoholic  solutions  of  organic  acids  (preferably  picric  acid),  the  successive  temper- 
ing to  dark  yellow  leaves  the  ferrous  sulphide  blue  and  the  manganese  sulphide  a  dull 
whitish.  The  reaction  appears  to  be  well  adapted  for  distinguishing  between  the 
sulphides. 

Etching  Wrought  Iron.  —  The  polished  samples  should  be  immersed  for  10  or 
15  seconds  in  a  10  per  cent  alcoholic  solution  of  nitric  acid  or  for  30  seconds  in  a  5 
per  cent  alcoholic  solution  of  picric  acid,  washed  in  alcohol  and  dried  with  the 
usual  precautions.  Concentrated  nitric  acid  may  also  be  used. 

The  etching  treatment  should  have  outlined  the  joints  between  the  ferrite  grains 
clearly  and  sharply.  If  the  structure  lacks  clearness  it  is  safe  to  infer  that  the  etch- 
ing was  not  properly  done.  In  that  case  the  sample  should  be  rubbed  a  few  times  on 
the  chamois  leather  block  and  again  examined  without  repeating  the  etching.  If 
the  structure  remains  ill-defined,  rub  the  specimen  a  minute  or  two  on  the  rouge 
block  or  disk,  wash,  dry,  and  repeat  the  etching  treatment  until  satisfactory  results 
are  obtained. 


46  CHAPTER   II  —  MANIPULATIONS 

Should  the  boundaries  of  the  ferrite  grains  appear  too  faint,  the  etching  treat- 
ment should  be  repeated  without  repolishing,  so  as  to  etch  these  lines  more  deeply. 

As  the  usual  purpose  of  the  microscopical  examination  of  samples  of  wrought 
iron  is  to  ascertain  the  quantity  and  mode  of  occurrence  of  the  slag  and  the  dimen- 
sions of  the  ferrite  grains,  it  is  not  generally  desired  to  etch  the  sample  so  deeply  that 
some  of  the  grains  become  deeply  colored,  still  less  that  etching  pits  begin  to  appear. 

As  an  experiment,  however,  it  is  advisable  to  etch  samples  of  wrought  iron  grad- 
ually so  that  the  different  stages  of  the  structure  may  be  clearly  seen:  (1)  before 
etching:  slag  fibers  and  a  brilliant  structureless  matrix,  (2)  after  a  slight  etching: 
ferrite  grains  sharply  defined  but  remaining  uncolored  or  but  slightly  colored,  (3)  after 
a  deeper  etching:  some  of  the  ferrite  grains  deeply  colored,  and  (4)  after  a  still  deeper 
etching:  small  cubic  etching  pits  beginning  to  appear. 

The  production  of  these  etching  pits,  however,  is  often  a  troublesome  and  uncer- 
tain operation.  Heyn  recommends  for  that  purpose  etching  with  double  chloride  of 
copper  and  ammonium,  others  (Stead)  "a  sufficiently  long  immersion  in  lukewarm 
20  per  cent  sulphuric  acid,  followed  by  cleaning  in  nitric  acid,"  or  again  (Le  Chatelier) 
an  acidulated  solution  of  ferric  chloride. 

Etching  Pearlitic  Steels.  —  Pearlitic  steels  should  be  etched  with  the  reagents 
already  indicated,  namely,  nitric  acid  and  alcohol,  picric  acid  and  alcohol  or  concen- 
trated nitric  acid,  the  manipulations  being  the  same  as  those  prescribed  for  etching 
wrought  iron.  An  immersion  of  10  seconds  in  nitric  acid  or  of  20  seconds  in  picric 
acid  generally  suffices.  The  contrast  between  the  bright  ferrite  or  brilliant  cementite 
and  the  dark  areas  of  pearlite  should  be  very  marked.  If  considerable  free  ferrite  is 
present  its  grains  should  appear  like  a  delicate  but  distinct  network.  The  absence 
of  this  network  is  generally  due  to  too  slight  an  etching.  Under  high  powers  the 
pearlite  should  appear  distinctly  laminated,  although  it  is  not  always  possible  to 
bring  out  the  structure  of  every  particle  of  that  constituent. 

Etching  Sorbitic  Steel.  —-  The  etching  of  sorbitic  steel  is  similar  to  that  of  pearlitic 
steel  both  in  regard  to  reagents  used  and  manipulations  with  the  exception  that 
sorbite  generally  etches  somewhat  more  quickly  than  pearlite.  Immersions  of  7 
or  8  seconds  duration  in  nitric  acid  or  of  15  seconds  in  picric  acid  generally  suffice. 
The  sorbitic  areas  will  have  a  granular  but  ill-defined  structure. 

Etching  Troostitic  Steel.  —  Troostite  is  etched  more  quickly  and  is  more  intensely 
colored  than  any  other  constituent  of  steel.  For  that  reason  very  short  immersions 
are  advisable,  namely  2  or  3  seconds  in  nitric  acid  or  5  seconds  in  picric  acid  or  better 
still  longer  immersions  in  weaker  acids  (5  per  cent  nitric  acid  or  2  per  cent  picric 
acid).  Such  short  etchings  do  not  as  a  rule  bring  out  the  structure  of  the  martensite 
which  generally  accompanies  troostite,  it  being  left  white  and  structureless.  To 
develop  its  structure  deeper  etchings  are  required. 

Etching  Martensitic  Steel.  —  Martensite  is  colored  more  quickly  than  pearlite 
and  sorbite  but  more  slowly  than  troostite,  hence  the  immersions  of  martensitic 
samples  in  the  etching  baths  should  be  of  relatively  short  duration,  5  seconds  in  nitric 
acid  or  10  seconds  in  picric  acid.  The  acicular  or  zigzag  structure  of  martensite 
should  be  brought  out,  but  high  power  may  be  needed  to  resolve  it. 

Etching  Austenitic  Steel.  —  Austenite  does  not  occur  in  commercially  treated 
carbon  steel  but  may  be  produced  by  some  drastic  treatments  as  later  explained.  It 
also  occurs  normally  in  certain  alloy  steels,  as  for  instance  in  Hadfield  manganese  steel. 
When  present  in  carbon  steels  its  structure  may  be  revealed  by  short  immersions  in 


CHAPTER   II  —  MANIPULATIONS  47 

nitric  or  picric  acid.  It  generally  remains  bright  while  the  martensite  often  accom- 
panying it  is  darkly  colored.  Some  writers  claim,  however,  that  it  may  sometimes 
be  colored  darker  than  martensite.  In  the  absence  of  much  martensite  its  structure 
is  polyhedral.  Martensite  is  readily  identified  through  its  acicular  or  zigzag  occur- 
rence. The  etching  of  austenitic  alloy  steels  is  dealt  with  in  the  following  paragraph. 

Etching  Alloy  Steels.  —  While  the  reagents  and  methods  used  for  the  etching  of 
ordinary  carbon  steels  are  frequently  effective  for  the  etching  of  alloy  steels  especially 
for  those  which  after  slow  cooling  become  pearlitic,  some  of  them  are  refractory  to 
these  treatments  much  longer  immersions  being  required  to  bring  out  their  structures 
while  in  a  few  instances  the  ordinary  methods  must  be  deeply  modified. 

According  to  some  manganese  steels  should  be  etched  deeply  in  picric  or  nitric 
acid,  washed  in  water  and  dried  without  any  attempt  at  removing  the  dark  colored 
films  covering  the  surfaces  of  the  specimens. 

High  speed  steel  may  be  etched  with  nitric  or  picric  acid  alcoholic  solutions  of  or- 
dinary strength  but  the  time  needed  varies  between  a  few  minutes  and  more  than  an 
hour  according  to  the  thermal  treatment  to  which  the  specimens  have  been  sub- 
jected, high  heating  followed  by  quick  cooling  generally  requiring  long  immersions. 
In  the  author's  laboratory,  Mr.  M.  Yatsevitch  has  developed  a  very  useful  method 
for  high  speed  steel.  Ten  cubic  centimeters  of  commercial  hydrogen  peroxide  are 
mixed  with  20  cubic  centimeters  of  a  10  per  cent  solution  of  sodium  hydrate  in 
water.  The  steel  samples  are  immersed  in  this  solution  for  10  to  12  minutes,  washed 
first  with  water  and  twice  with  alcohol  and  dried,  preferably  with  a  blast.  The  re- 
agent should  be  prepared  fresh  every  day.  After  the  treatment  indicated  above  the 
polished  samples  remain  bright  and  their  matrix  structureless  but  the  free  carbide 
containing  tungsten  is  found  to  have  been  colored  dark.  While  it  is  probable  that 
other  special  carbides  such  as  carbide  of  molybdenum  would  be  likewise  colored,  no 
other  constituent  appears  to  be  acted  upon  by  this  reagent,  not  even  iron  carbide 
(cementite) .  The  method  affords  a  means  of  bringing  out  clearly  the  amount  of  free 
carbide  in  high  speed  steel  and  of  distinguishing  between  it  and  other  constituents. 

Etching  Cast  Iron.  —  The  etching  of  polished  samples  of  cast  iron  is  conducted 
in  every  respect  like  the  etching  of  steel  specimens.  To  bring  out  clearly  the  phosphide 
eutectic  and  to  prevent  its  being  mistaken  for  pearlite  areas  or  vice  versa  the  heat 
testing  method  devised  by  Stead  is  very  useful.  Polished  samples  of  gray  cast  iron 
should  generally  be  first  examined  before  etching,  the  distribution,  size,  and  form  of 
the  graphite  particles  being  then  more  clearly  revealed. 

Etching  for  Macrostructure.  —  To  reveal  the  macrostructure,  that  is  the  structure 
visible  with  the  naked  eye,  and  also  to  detect  local  segregations,  especially  of  phos- 
phorus, Portevin  recommends  the  immersion  of  the  polished  samples  for  3  min- 
utes in  double  chloride  of  copper  and  ammonium  (10  grams  of  the  double  chloride  in 
120  cubic  centimeters  of  water)  followed  by  an  immersion  of  30  minutes  in  10  per  cent 
nitric  acid.  Heyn  and  Stead  mentioned  immersion  of  2  minutes  in  a  10  per  cent  solu- 
tion of  cupric  ammonium  chloride  in  water,  the  deposits  of  copper  being  removed 
under  a  stream  of  water,  while  gently  rubbing  the  surface  with  the  fingers  or  a  piece 
of  chamois  leather.  Similar  results  are  obtained  by  etching  in  20  per  cent  nitric  acid 
in  water,  or  in  dilute  hydrochloric  or  sulphuric  acid. 

ti.minhtnli<in .  -The  prepared  sample  should  be  suspended  to  the  magnetic 
specimen  holder  previously  described  in  such  a  way  as  to  expose  to  view  as  much  as 
possible  of  its  surface.  The  source  of  light  and  condensers  should  be  adjusted  so 


48  CHAPTER  II  —  MANIPULATIONS 

that  a  beam  of  light  of  suitable  size  enters  the  vertical  illuminator;  the  light  beam 
should  cover  a  little  more  than  the  aperture  of  the  illuminator.  A  2-in.  (5X)  eye- 
piece and  a  16-mm.  (%-in.)  objective  will  be  generally  a  satisfactory  combination  for 
the  first  examination.  The  image  of  the  specimen  should  be  focused  roughly  by  the 
rack  and  pinion  motion  of  the  stage,  and  the  milled  head  of  the  vertical  illuminator 
turned  tentatively  and  gently  right  and  left  until  the  sample  appears  brightly  and 
uniformly  lighted.  The  object  should  now  be  brought  to  a  sharp  focus  by  means  of 
the  fine  adjustment. 

As  previously  stated,  if  the  structure  lacks  clearness  it  is  safe  to  infer  that  the 
etching  was  not  properly  done.  In  that  case  the  sample  should  be  rubbed  a  few 
times  on  the  chamois  leather  block  and  again  examined  without  repeating  the  etch- 
ing. If  the  structure  remains  ill-defined,  rub  the  specimen  a  minute  or  two  on  the 
rouge  block  or  disk,  wash,  dry,  and  repeat  the  etching  treatment  until  satisfactory 
results  are  obtained. 

Examinations  with  high-power  objectives  (4  mm.  or  higher)  require  greater  care 
and  more  delicate  manipulations.  It  is  especially  for  high-power  work  that  the  plain 
disk  illuminator  is  superior  to  the  prism  type. 

The  specimens  should  be  examined  systematically  over  their  entire  surfaces,  for 
which  purpose  a  mechanical  stage  is  invaluable,  in  order  to  judge  of  the  uniformity 
or  lack  of  uniformity  of  its  structure.  It  should  be  borne  in  mind  that  the  portions 
of  heat  treated  specimens  corresponding  to  the  outside  of  the  original  samples  are 
very  likely  to  be  decidedly  lower  in  carbon  owing  to  the  decarbonizing  atmosphere 
generally  present  in  heating  furnaces.  It  is  often  advisable  to  first  examine  the 
specimens  before  etching  for  the  presence  of  slag  or  other  enclosures,  graphitic  or 
temper  carbon,  minute  blow  holes,  etc.,  these  being  more  readily  detected  in  the 
unetched  condition. 

In  studying  the  structure  of  forged  or  rolled  samples,  specimens  should  generally 
be  prepared  corresponding  to  their  cross  sections  as  well  as  to  their  longitudinal  sec- 
tions, in  order  to  detect  any  possible  distortion  or  orientation  of  structure  caused  by 
the  forging  or  rolling. 

Photomicrography.  —  After  having  selected  the  spot  to  be  photographed,  light- 
tight  connection  should  be  established  between  the  microscope  and  the  camera  (in 
the  case  of  the  inverted  metalloscope  the  camera  and  microscope  are  permanently 
connected)  and  the  light  carefully  adjusted  so  as  to  obtain  on  the  screen  of  the 
camera  as  bright  and  even  an  illumination  as  possible.  The  image  should  now  be 
focused  as  sharply  as  possible,  using  a  focusing  cloth  if  necessary  and  gently  turning 
the  fine  adjustment  screw  of  the  stand.  A  focusing  glass  may  be  used  with  great 
advantage  for  this  operation  and  is  of  special  importance  when  photographing  with 
high-power  objectives.  It  should  be  placed  on  the  plain  glass  circle  which  occupies 
the  center  of  the  screen  of  the  camera,  and  the  image  focused  while  being  viewed 
through  this  lens.  By  this  means  we  magnify  the  image  formed  upon  the  camera 
screen,  and  are  therefore  able  to  focus  it  more  sharply  in  its  finer  details.  Consid- 
erable light,  however,  is  lost  and  the  object  will  often  appear  but  dimly  lighted. 
The  rule  is  to  secure  the  clearest  possible  image  while  working  tentatively  the  fine 
adjustment  in  both  directions,  bearing  in  mind  that,  at  its  best,  the  image  may 
appear  blurred  and  dimly  lighted. 

An  ordinary  eye-piece  may  be  used  in  place  of  a  focusing  glass  with,  in  many 
cases,  satisfactory  results. 


CHAPTER   II  —  MANIPULATIONS 


49 


Exposure.  —  After  the  image  has  been  properly  illuminated  and  focused  the  sen- 
sitive plate  should  be  introduced  and  exposed,  with  the  ordinary  precautions,  for  a 
suitable  length  of  time.  The  required  time  of  exposure  will  vary  according  to  (a) 
the  kind  of  photographic  plate  used  and  (6)  the  amount  and  nature  of  light  reaching 
the  plate,  which  in  turn  will  depend  upon  (1)  the  nature  of  the  prepared  surface, 
especially  its  power  to  reflect  light,  (2)  the  kind  of  illumination  used,  (3)  the  position 
of  the  diaphragm  or  diaphragms  controlling  the  amount  of  light  allowed  to  reach  the 
plate,  (4)  the  kind  of  light  niters  used,  if  any,  (5)  the  resolving  and  magnifying  powers 
of  the  combination  of  objective  and  eye-piece  used,  and  (6)  theHistance  between  the 
screen  of  the  camera  and  the  object. 

Specimens  which  after  etching  remain  quite  bright  naturally  reflect  more  light 
and  consequently  require  for  their  photography  a  shorter  time  than  duller  specimens. 
Generally  speaking  the  higher  the  magnification  the  less  light,  hence  the  longer  the 
exposure.  The  use  of  colored  screens  or  solutions  (light  filters)  as  a  rule  lengthens  the 
exposure  considerably.  By  placing  the  screen  of  the  camera  at  a  greater  distance  from 
the  object  (i.e.  by  extending  the  bellows  of  the  camera)  the  magnification  is  increased 
but  with  accompanying  loss  of  light  and,  therefore,  increased  length  of  exposure. 

Using  a  rather  slow  plate,  no  screens,  a  combination  of  objective  and  eye-piece 
yielding  a  magnification  of  100  diameters  at  a  distance  of  24  to  30  inches  from  the 
object,  the  diaphragm  being  wide  open,  the  exposure  for  most  iron  and  steel  samples 
would  vary  between  a  fraction  of  a  second  with  a  powerful  arc  lamp  and  some  10  to 
20  minutes  with  a  welsbach  lamp.  The  use  of  higher  magnifications,  of  screens,  and 
the  closing  of  the  diaphragm  may  lengthen  the  exposure  to  such  an  extent  as  to  re- 
quire one  minute  or  more  with  an  arc  lamp  and  one  hour  or  more  with  a  welsbach  lamp. 

The  use  of  sources  of  light  of  greater  intensity  than  the  welsbach  mantle  but 
less  intense  than  the  electric  arc,  such  as  the  Nernst  lamp,  the  acetylene  lamp,  or 
the  oxy-hydrogen  lamp,  calls  for  exposures  of  intermediate  lengths  between  the  two 
extremes  considered  in  the  preceding  paragraph. 

When  using  the  photomicrographic  apparatus  Figure  33,  Seeds  orthochromatic 
"L"  plates,  a  special  green  screen,  the  illuminator  diaphragm  being  wide  open,  the 
shutter  diaphragm  closed  at  7.5,  and  the  distance  between  the  upper  lens  of  the  eye- 
piece and  the  screen  of  the  camera  being  385  mm.,  the  following  exposures  in  seconds 
for  various  combinations  of  objectives  and  eye-pieces  will  generally  yield  excellent 
results : 

EXPOSURE   AND   MAGNIFICATION   TABLE 
EYE-PIECES  (magnification  when  used  as  magnifiers) 
5  x  6.4  x  7.5  X  10  x  12.5  x  15  x 


n 

G         *?*? 

5 

„ 

g 

11 

13         15 

a-1 

(25) 

(35) 

(40) 

(50) 

(65)      .  (80) 

=H  O  C 

b1*-  ~    ifi 

7 

10 

12 

15 

IS         20 

(75) 

(100) 

(115) 

(150) 

(190) 

(235) 

--5  c 

15 

20 

25 

30 

35 

40 

3  "•      1 
0" 

(310) 

(425) 

(500) 

(650) 

(825) 

(1000) 

EXPOSURE  IN  SECONDS 
Numbers  between  parentheses  indicate  approximate  magnification  in  diameters 


50  CHAPTER  II  —  MANIPULATIONS 

Diaphragms  and  Shutters.  —  It  is  sometimes  advantageous  to  be  able  to  control 
the  pencil  of  light  entering  the  illuminator  with  a  view  of  securing  sharper  definition. 
To  that  effect  an  iris  diaphragm  suitably  mounted  should  be  placed  between  the 
condensing  lens  or  lenses  and  the  vertical  illuminator.  Some  sort  of  an  automatic 
shutter  is  convenient  to  control  the  exposure  of  the  plates.  This  shutter  may  ad- 
vantageously be  combined  with  the  iris  diaphragm.  Instead  of  being  placed  between 
the  source  of  light  and  the  vertical  illuminator,  diaphragms  and  shutters  are  some- 
times inserted  between  the  camera  and  the  microscope.  The  best  disposition  con- 
sists in  placing  an  iris  diaphragm  between  the  condensing  lenses  and  the  vertical 
illuminator,  thus  controlling  the  amount  of  light  entering  the  latter,  and  another 
diaphragm  combined  with  automatic  shutter  between  the  camera  and  microscope 
tube. 

Monochromatic  Light.  —  The  different  sources  of  light  used  for  microscopical 
work  yield  white  light  and  since  the  correction,  even  of  apochromatic  objectives,  for 
chromatic  aberration  is  never  perfect,  it  is  evident  that  the  use  of  monochromatic 
light,  i.e.  light  of  one  wave  length,  is  theoretically  preferable,  especially  for  photo- 
graphing. 

Monochromatic  light  may  be  obtained  in  two  ways:  (a)  by  using  a  source  of  light 
actually  monochromatic,  and  (6)  by  causing  white  light  to  pass  through  colored  glass 
screens  or  colored  solutions  (light  filters),  preventing  the  passage  of  some  undesirable 
rays.  The  mercury  arc  lamp  yields  a  nearly  monochromatic  light  and  has  been  tried 
by  Le  Chatelier  with  satisfactory  results.  It  seems  more  convenient,  however,  when 
monochromatic  light  is  wanted,  to  use  light  filters  of  suitable  colors,  in  which  case 
colored  glass  screens  will  be  found  easier  to  manipulate  than  glass  cells  containing 
colored  solutions.  The  author  recommends  a  special  green  screen  supplied  by  Nachet 
of  Paris  or  its  equivalent. 

Photographic  Plates.  —  The  use  of  orthochromatic  plates  together  with  a  suitable 
screen  is  recommended. 

Development.  —  Formula  and  directions  accompany  each  box  of  plates  and  the 
student  could  not  do  better  than  to  follow  them  faithfully. 

Printing.  —  Any  printing  out  or  developing  paper  may  be  used,  the  printing,  de- 
veloping, or  toning  being  conducted  in  the  usual  way.  Drying  on  ferrotype  plates 
affords  a  quick  means  of  finishing  the  prints  and  giving  them  a  satisfactory  luster. 
It  is  recommended  to  trim  the  prints  round,  2  to  2^  inches  in  diameter,  by  means 
of  a  margin  trimmer  and  suitable  circular  forms,  as  this  will  give  them  a  very  neat 
appearance. 


CHAPTER  III 

APPARATUS  AND  MANIPULATIONS   (Continued) 

The  apparatus  used  by  the  author  and  his  methods  have  been  described  in 
Chapters  I  and  II.  Other  instruments  and  manipulations  are  here  briefly  mentioned. 

POLISHING  AND  POLISHING   MACHINES 

Sorby  in  his  pioneer  work  polished  his  samples  on  emery-papers  of  increasing 
fineness  followed  by  rubbing  with  tripoli,  crocus,  or  Water-of-Ayre  stone,  and  finally 
with  jeweler's  rouge.  Emery-papers  are  still  used,  but  for  quick  polishing  they 
are  often  replaced  by  emery-powders  spread  wet  on  revolving  wheels;  the  author 
has  retained  the  use  of  tripoli  powder  for  the  treatment  preceding  the  final  polish- 
ing but  others  now  prefer  specially  prepared  flour-emery  or  diamantine;  jeweler's 
rouge  is  still  widely  used  for  the  final  treatment,  although  some  prefer  specially  pre- 
pared alumina,  as  first  suggested  by  Le  Chatelier. 

In  1904  Osmond's  polishing  method  consisted  in  roughing  off  with  emery  and 
polishing  with  rouge.  Emery-papers  of  increasing  fineness  were  stretched  over  glass 
plates.  The  papers  used  were  prepared  by  mixing  with  water  levigated  "120 
minutes"  emery1  and  collecting  the  deposits  formed  at  the  end  of  increasing  periods 
of  time  in  precipitating  glasses.  The  classified  powders,  after  drying,  were  mixed 
with  a  mucilage  of  albumen  (made  up  of  72  cubic  centimeters  of  albumen  and  28 
cubic  centimeters  of  water  beaten  to  a  froth  and,  after  12  hours,  strained  through  a 
fine-meshed  sponge)  anil  spread  on  paper  of  the  best  quality.  Osmond  also  pre- 
pared his  own  rouge  by  calcining  copperas  at  as  low  a  temperature  as  possible  and 
separating  the  finest  product  by  levigation.  The  rouge  was  spread  on  a  piece  of 
cloth  stretched  over  the  cast-iron  table  of  a  small  horizontal  polishing  machine  and 
used  wet. 

In  1900  Le  Chatelier's  method  of  polishing  specimens  of  iron  and  steel  previously 
rubbed  upon  emery-papers,  including  the  finest  grades,  consisted  in  rubbing  them 
successively  (1)  on  emery-paper  prepared  with  albumen,  according  to  Osmond,  with 
the  deposit  obtained  in  between  a  quarter  of  an  hour  and  one  hour  in  the  ammoniacal 
washing  of  flour-emery,  (2)  on  a  felt  disk  covered  with  some  soap  paste  prepared 
with  the  deposit  of  alumina  or  of  emery,  obtained  in  between  one  and  three  hours, 
(3)  on  a  flat  disk  made  of  wood,  metal,  or  ebonite,  covered  with  cloth,  velvet,  or 
leather  strongly  glued  upon  it;  upon  this  covering  the  soap  preparation,  obtained 
with  the  deposit  of  alumina  after  twenty-four  hours,  was  spread.  The  last  two  disks 
were  rotated  by  some  mechanical  devices  producing  groat  speed.  All  disks  must  be 
frequently  moistened  with  a  brush  or  sponge. 

1  By  "120  minutes"  emery  is  meant  emery  which  has  taken  120  minutes  to  settle  in  a  vessel  of 
water  of  certain  dimensions. 

51 


52 


CHAPTER  III  —  APPARATUS  AND  MANIPULATIONS 


According  to  Gcerens,  Le  Chatelier's  method  in  1908  consisted  in  the  use  of  (1) 
small  sheets  of  French  emery-paper,  Hubert  grades  IG  and  00  on  ground  glass  plates, 
(2)  flannel  stretched  over  glass  covered  with  "one  minute"  emery  previously  passed 
through  a  fine  sieve  (1200  meshes  per  sq.  cm.),  some  soap  solution  being  also  poured 
over  the  cloth,  (3)  a  similar  support  covered  with  "120  minutes"  emery  previously 
passed  through  a  very  fine  sieve  (2600  meshes  per  sq.  cm.)  and  washed,  and  (4)  a 
vertically  revolving  brass  disk  covered  with  flannel  and  washed  alumina.  The  fine 
alumina  mixed  with  water  and  soap  solution  may  be  sprayed  on  the  disk  by  means 
of  the  sprayer  shown  in  Figure  46. 

The  preparation  of  fine  alumina  powder  for  the  final  polishing  of  iron  and  steel 
samples  was  first  described  by  Le  Chatelier  in  1900.  The  method  used  is  that  em- 


Fig.  46.  — •  Sprayer  for  emulsified  alu- 
mina.    (Gcerens.) 


Fig.  47.  — •  Pipette  for  the  levigation 
of  alumina.     (Gcerens.) 


ployed  by  Schlcesing  for  the  analysis  of  kaolins.     The  following  description  of  Le 
Chatelier's  manipulation  is  from  Gcerens  (1908). 

The  purest  precipitated  alumina,  from  ammonia  alum,  is  passed  through  a  sieve, 
of  2600  meshes  per  sq.  cm.,  and  100  grains  of  it  in  300  c.  c.  of  distilled  water  are  trit- 
urated in  a  mill  for  three  hours.  The  whole  is  then  poured  into  a  liter  flask,  well 
shaken,  and  about  200  c.  c.  pipetted  off  into  a  flask  closed  with  a  rubber  stopper. 
To  this  are  added  1800  c.  c.  of  distilled  water  and  2  c.  c.  of  concentrated  nitric  acid 
(1.4  sp.  gr.),  the  mixture  well  shaken,  and  allowed  to  settle;  the  settling  is  complete 
in  a  short  time  (about  two  hours).  The  clear  supernatant  liquid  is  siphoned  off  with 
an  S-shaped  siphon;  with  careful  manipulation  this  is  possible  to  the  extent  of  ,''„ 
of  the  total  amount.  The  liquid  drawn  off  is  replaced  by  distilled  water,  the  mixture 
well  shaken  several  times,  and  allowed  to  settle  again,  after  which  the  wash  water  is 
again  drawn  off  as  before.  This  is  repeated  three  or  four  times  more.  At  last  the 
supernatant  liquid  remains  milky  for  a  whole  day,  which  is  an  indication  of  the  per- 


CHAPTER  III  —  APPARATUS  AND   MANIPULATIONS  53 

feet  removal  of  acid.  Finally,  distilled  water  is  added  for  the  last  time  up  to  about 
2  liters,  the  mixture  thoroughly  shaken,  and  the  alumina  separated  from  the  liquid 
in  the  apparatus  shown  in  Figure  47.  A  pipette  a  of  about  500  c.  c.  capacity  is  drawn 
out  below  to  an  opening  of  about  3  mm.  internal  diameter.  The  alumina  is  prevented 
from  clinging  by  giving  an  inclination  of  at  least  J^  to  the  sloping  sides  of  the  tube. 
The  piece  b  is  connected  to  the  water  pump.  The  end  (of  the  pipette)  is  dipped 
into  the  vessel  containing  the  emulsified  alumina,  and  the  pipette  sucked  full,  where- 
upon the  opening  b  is  closed  with  a  screw  cap  so  far  that  one  dropj-uns  out  about  every 
fifteen  seconds.  The  material  obtained  during  the  first  quarter  of  an  hour  is  very 
heterogeneous  and  still  scratches  the  surface  of  the  section  markedly,  so  that  it  can- 
not be  used.  After  a  quarter  of  an  hour  has  expired  the  tap  is  closed  and  the  alu- 
mina allowed  to  settle  completely.  After  three  hours  the  material  is  placed  in  the  flask 
A  (Fig.  46)  provided  with  a  spraying  arrangement.  Soap  solution1  is  added  and  the 
mixture  diluted  with  distilled  water.  The  material  thus  prepared  is  ready  for  use, 
and  is  suitable  for  steel  and  pig  iron.  The  residue  which  settles  in  3  to  24  hours  is 
treated  similarly,  and  serves  for  polishing  softer  materials  (iron,  copper,  etc.).  The 
portion  which  still  remains  in  suspension  after  24  hours  is  too  fine  and  is  poured  away. 

The  same  method  has  been  applied  to  commercial  flour-emery,  oxide  of  chromium, 
and  oxide  of  iron,  but  the  resulting  products  are  far  from  being  as  satisfactory  as  the 
alumina  powders. 

Revillon  has  recently  described  a  rapid  method  of  preparing  alumina  suitable  for 
polishing.  A  certain  amount  of  alumina  is  suspended  in  a  large  volume  of  water, 
well  shaken,  and  allowed  to  stand  for  five  minutes.  The  liquid  is  then  siphoned  off 
and  with  the  particles  of  alumina  still  held  in  suspension  may  be  used  for  polishing. 
To  obtain  finer  powders  15  to  20  grams  of  finely  ground  alumina  should  be  mixed 
with  one  liter  of  distilled  water,  shaken,  and  allowed  to  settle  five  minutes.  Most  of 
the  liquid  is  then  siphoned  off,  transferred  into  another  vessel,  and  allowed  to  stand 
fifteen  minutes;  the  decantation  is  repeated,  etc.,  a  clearer  liquid,  that  is  one  holding 
finer  particles  in  suspension,  being  obtained  every  fifteen  minutes.  The  final  liquid, 
from  which  no  powder  is  deposited,  may  be  used  for  the  finest  polishing,  the  inter- 
mediate products  for  rougher  work. 

Robin  has  described  the  preparation  of  alumina  powder  by  a  method  based  upon 
the  catalytic  action  of  mercury  in  causing  the  oxidation  of  pure  aluminum.  Strips 
of  aluminum  are  immersed  in  mercury  for  a  short  time  and  then  exposed  to  moist 
air  when  the  small  amount  of  mercury  they  have  absorbed  causes  the  oxidation  of 
the  metal,  growth  of  A1203  taking  place,  the  increase  of  which  is  visible  with  the  naked 
eye.  This  alumina  can  be  readily  detached  and  as  a  fine  powder  may  be  used  for 
polishing.  Robin  claimed  for  his  method  the  advantages  of  greater  simplicity  and 
lower  cost. 

In  1900  Stead  recommended  for  polishing  iron  and  steel  samples  the  use  of  emery- 
papers,  Hubert  grades  Nos.  0,  00,  and  000,  followed  by  rubbing  with  one  grain  of 
diamantine  powder2  spread  wet  over  a  smooth  black  cloth  and,  for  final  treatment, 
gold  rouge  used  dry  on  chamois  leather  or,  for  finer  structures,  wet  on  parchment  or 
kid  leather.  He  used  a  simple,  hand  polishing  machine  in  which  one  block  at  a  time 
was  made  to  rotate  horizontally  (Fig.  48). 

1  The  soap  solution  is  prepared  by  dissolving  pure  (Venetian)  soap  in  hot  water  and  filtering 
through  a  filter  paper  into  a  flask.    After  cooling  the  solution  should  be  sirupy. 

2  Diamantine  powder  consists  of  pure  alumina  and  is  used  by  jewelers  for  polishing  steel. 


54 


CHAPTER   III  —  APPARATUS   AND   MANIPULATIONS 


More  recently  he  has  described  as  follows  his  polishing  manipulation:  "The 
surfaces  are  made  smooth  by  filing  or  grinding  on  coarse  emery-cloth,  and  afterwards 
on  progressively  finer  emery  paper,  until  the  finest  French  paper  000  is  reached. 


o 

12 


o 
P. 

-a 
a 
a 

W 


38 


Fine  polishing  is  then  conducted  on  a  wheel  either  having  horizontal  or  vertical  mo- 
tion, and  covered  with  two  layers  of  thick  khaki  cloth  between  which  a  layer  of  dia- 
rnantine  powder  or  other  specially  prepared  polishing  powder  is  placed."  Water  is 


CHAPTER   III  — APPARATUS   AXD   MANIPULATIONS 


55 


run  on  the  prepared  surface  of  the  polishing  block  the  speed  of  which  should  be  at 
least  1000  revolutions  per  minute.  "Pressure  must  be  applied  strongly  at  first,  and 
then  be  gradually  reduced  until  finally  little  more  than  the  weight  of  the  specimen 
and  the  fingers  pressing  it  press  on  the  polishing  surface.  With  practice  a  specimen 
can  be  prepared  in  10  minutes." 

A  foot  polishing  machine  also  designed  by  Stead  is  shown  in  Figure  491  and  a 
larger  one  to  be  run  by  power  in  Figure  50.    In  these  machines  brass  disks  carrying 


Fig.  49.  —  Foot  power  polishing  machine. 
(Stead.) 


Fig.  50.  —  Multiple  polishing  machine.     (Stead.) 


conical  wooden  blocks  are  attached  to  vertical  spindles  and  driven  from  below. 
Emery-papers  are  fastened  to  some  of  these  blocks  by  means  of  brass  rings  slipping 
over  them  while  others  are  covered  with  cloth  in  a  similar  way. 

The  block  for  fine  polishing  is  covered  with  a  double  layer  of  cloth  between  which 
is  placed  specially  prepared  calcined  alumina  in  fine  powder.  Jeweler's  diamantine 
powder  answers  admirably.  Fine  polishing  is  always  done  on  cloth-covered  blocks, 
kept  continually  moist  with  clean  water.  Only  the  finest  parts  of  the  powders  find 

1  A  similar  machine  is  supplied  with  electric  motor  attached. 


56 


CHAPTER  III  —  APPARATUS  AND   MANIPULATIONS 


their  way  through  the  porous  cloth.  Clamps  are  provided  for  holding  the  samples 
against  the  revolving  disks.  The  central  vessel  contains  the  water  needed  for  wet 
polishing,  a  small  tap  projecting  over  each  disk.  The  excess  water  is  caught  by 
brass  water  guards  and  discharged  into  a  trough  below  the  level  of  the  disks.  These 
machines  are  made  by  Carling  and  Sons  of  Middlesborough,  England. 


Fig.  51.  —  Foot  power  polishing  machine.     (P.  F.  Dujardin  and  Co.) 


Martens,  according  to  Gcerens  (1908),  used  vertically  rotating  disks  upon  which 
were  pasted  emery-papers,  Hubert  brand,  grades  3,  2,  1G,  1M,  IF,  0,  and  00  and,  for 
final  treatment,  levigated  jeweler's  rouge  on  cloth.  The  disks  made  400  revolutions 
per  minute.  The  average  time  needed  to  polish  a  specimen  varied  between  1J^  and 
2  hours. 

Gulliver  (1908)  recommends  for  polishing  the  use  of  emery-papers  grades  No.  1, 


CHAPTER   III  —  APPARATUS   AND   MANIPULATIONS 


57 


0,  and  00  on  hard  wood  or  plate  glass  and  for  final  treatment  the  finest  rouge  or  dia- 
mantine  powder  on  cloth  stretched  over  hard  wood. 

The  polishing  machines  shown  in  Figures  51  and  52  are  made  by  P.  F.  Dujardin  of 
Dusseldorf.    It  will  be  noted  that  one  side  only  of  the  disks  is  utilized.    A  machine 


Fig.  52.  —  Polishing  motor.     (P.  F.  Dujardin  and  Co.) 


like  the  one  of  Figure  52  is  also  made  for  belt  driving.  A  similar  machine  is  supplied 
by  Pellin  of  Paris. 

Sexton  describes  the  polishing  machine  Figure  53  made  by  Baird  and  Tatlock. 
Its  construction  is  obvious. 

A  simple  polishing  machine  consisting  of  an  horizontally  revolving  disk  (Fig.  54) 
was  described  in  1899  by  Ewing  and  Rosenhain.  A  is  the  spindle  of  an  electric  motor 
carrying  a  small  driving  disk  B,  fitted  with  a  rubber  ring  to  increase  the  driving  fric- 


58  CHAPTER   III  —  APPARATUS   AND    MANIPULATIONS 

tion.  The  polishing  disk  C  has  a  vertical  axis  running  in  a  bearing  on  the  easting  D. 
The  under  side  of  the  polishing  disk  bears  upon  the  driving  wheel  B  and  takes  motion 
from  it. 

A.  Kingsbury  in  1910  describes  his  polishing  method.  He  prepares  his  support- 
ing blocks  by  pouring  paraffin  on  brass  disks.  After  solidifying  these  paraffin  blocks 
which  are  about  J^  inch  thick  and  8  inches  in  diameter  have  their  upper  face  dressed 
flat.  They  are  made  to  rotate  horizontally  in  a  suitable  machine  and  upon  them 
emery  of  increasing  fineness  and  finally  rouge  are  used  in  succession.  The  speed 
of  the  polishing  machine  is  200  revolutions  per  minute.  The  time  needed  to  polish  a 
sample  of  ordinary  steel  is  given  as  15  minutes. 

C.  Campbell  in  1902  described  his  polishing  operation  as  consisting  in  rubbing  the 
sample,  previously  filed  smooth,  successively  on  emery-cloth,  grades  0  and  00,  and  on 


Fig.  53.  —  Polishing  machine.     (Baird  and  Tatlock.) 

French  emery-papers,  grades  0,  00,  000,  and  0000.  The  specimen  is  then  polished  on 
broadcloth  or  chamois  leather  with  well  washed  rouge  and  water.  Some  workers, 
the  writer  says,  use  an  intermediate  stage  with  d'amantine  powder. 

C.  H.  Risdale  in  1899  described  his  polishing  operation  as  consisting  in  (1)  rough 
filing,  (2)  fine  filing,  (3)  rubbing  with  rough  commercial  emery-cloth  stretched  on  a 
board,  (4)  rubbing  with  fine  emery-cloth  stretched  on  a  board,  (5)  rubbing  on  fine 
specially  prepared  paper  on  disks  of  Stead's  polishing  machine,  (6)  rubbing  on  dia- 
mantine  on  cloth  stretched  on  disks  of  Stead's  machine,  (7)  rubbing  on  rouge  on 
washed  leather  similarly  mounted  or,  for  very  fine  work,  on  rouge  on  wetted  parch- 
ment. 

Guillet  in  1907  recommended  for  polishing  two  carborundum  wheels  and  two 
suitably  selected  emery-papers  and,  for  final  treatment,  alumina  on  cloth  stretched 
over  a  revolving  disk.  He  places  smooth  sheets  of  zinc  between  the  wooden  disks  of 
his  polishing  machines  and  the  polishing  cloths. 


CHAPTER  III  —  APPARATUS  AND   MANIPULATIONS 


59 


In  1901  Arnold  described  as  follows  a  quick  polishing  and  etching  method:  "Take 
two  pieces  of  hard  wood,  12"  X  9"  X  1",  planed  dead  smooth  on  one  side;  then  by 
means  of  liquid  glue  evenly  attach  to  the  smooth  faces  two  sheets  of  the  London 
Emery  Works  Co.'s  atlas  cloth  No.  0.  Allow7  the  glue  to  set  under  strong  pressure. 


Fig.  54.  —  Polishing  machine.     (Ewing  and  Rosenhain.) 

Next,  by  means  of  a  smooth  piece  of  steel,  rub  off  from  one  of  the  blocks  as  much  as 
possible  of  the  detachable  emery.  This  is  No.  2  block,  the  other,  necessarily,  No.  1 
block. 

''The  steel  section,  say  %  inch  thick  and  J/2  inch  diameter,  is  rubbed  for  1  minute 
mi  Xo.  1  block,  the  motion  being  straight  and  not  circular;  then,  for  the  same  time 


Fig.  55.  —  Polishing  machine.     (Wysor.) 

and  in  the  same  manner  rub  on  No.  2  block.  Next  place  the  bright  but  visibly 
scratched  sections  in  a  glass  etching  dish  3"  X  1"  X  Y^,' ,  and  cover  the  steel  with  nitric 
acid  sp.  gr.  1.20. 

"Watch  closely  until  in  a  few  seconds  the  evolved  gases  adhering  to  the  section 
change  from  pale  to  deep  brown  and  effervescence  ensues.     Then,  under  the  tap, 


60  CHAPTER  III  —  APPARATUS  AND  MANIPULATIONS 

quickly  wash  away  the  acid  and  for  a  minute  immerse  the  piece  in  a  second  dish  con- 
taining rectified  methylated  spirits.  Dry  the  section  by  pressing  it  several  times  on 
a  soft  folded  linen  handkerchief,  when  it  will  be  ready  for  examination.  The  struc- 


Fig.  56.  —  Polishing  machine.     (R.  Fuess.) 

ture  will  be  clearly  exhibited,  the  innumerable  fine  scratches  visible  before  etching 
having  virtually  vanished." 

Robin  polishes  his  samples  (1)  on  carborundum  or  hard  emery-wheel  (500  to  1000 
revolutions  per  minute),  (2)  on  fine  emery-wheel  (1000  revolutions),  (3)  by  hand  on 


Fig.  57.  —  Polishing  machine.     (Scientific  Materials  Co.) 

emery-papers  of  increasing  fineness,  and  (4)  with  jeweler's  rouge  and  water  on  wooden 
disk  (1000  revolutions). 

Professor  Wysor  has  designed  the  polishing  machine  shown  in  Figure  55  in  which 
three  polishing  blocks  are  made  to  revolve  simultaneously,  two  vertically  and  one 
horizontally. 


CHAPTER  III  —  APPARATUS  AND  MANIPULATIONS  61 

R.  Fuess  of  Steglitz  bei  Berlin  offers  the  polishing  motor  (Fig.  56),  in  which  but 
one  polishing  disk  revolves  at  a  time. 

The  Scientific  Materials  Company  of  Pittsburg,  Pennsylvania,  construct  the  ma- 
chine illustrated  in  Figure  57.  The  motion  is  reciprocating. 

Polishing  Small  Specimens.  —  To  make  possible  the  polishing  of  very  small  speci- 
mens or  for  preventing  the  rounding  of  the  edges  of  samples  of  moderate  size,  Le 
Grix  uses  small  brass  cylinders  which  he  fills  with  gum  lac  and  in  which  he 
presses  the  pieces  to  ba.polished  (chips,  cuttings,  etc.).  The  whole  is  then  polished 
as  if  it  were  a  solid  piece.  The  modus  operand!  is  as  follows :  The  brass  mounting  is 
heated  in  a  bunsen  burner  and  filled  with  gum  lac  used  in  the  form  of  sticks 
like  sealing  wax.  After  cooling  the  excess  gum  is  removed  with  a  file  to  a  level 
with  the  mount  and  the  specimens  pressed  into  the  filling  material  by  means  of  a 
preheated  metallic  blade.  When  the  specimens  are  very  small  the  polishing  opera- 
tion is  very  short  indeed  (Le  Chatelier  mentions  3  minutes).  The  etching  reagent 
may  be  applied  to  the  metallic  particles  by  means  of  a  glass  rod  thereby  preventing 
the  brass  mounting  and  even  the  gum  from  coming  in  contact  with  it  which  is  de- 
sirable since  alcoholic  solution  would  dissolve  the  latter.  The  same  material  may 
also  be  used  with  advantage  for  filling  up  small  cavities  such  as  blow  holes  occasionally 
present  in  samples  to  be  polished,  thereby  preventing  the  rounding  of  their  edges  or 
the  tearing  of  the  polishing  cloths. 


DEVELOPMENT  OF  THE  STRUCTURES 

The  methods  which,  in  common  with  many  workers,  the  author  has  found  most 
satisfactory  for  revealing  the  structure  of  polished  iron  and  steel  specimens  have  been 
described  in  Chapter  II.  Other  methods  have  been  used  that  should  be  mentioned. 

Polishing  in  Relief.  —  So-called  relief  polishing  has  been  used  successfully  by 
Sorby,  Martens,  Behrens,  and  especially  by  Osmond.  It  consists  in  rubbing  the 
specimen  on  a  soft,  yielding  support  with  some  suitable  polishing  powder,  the  softest 
constituents  being,  so  to  speak,  dug  out,  leaving  the  harder  ones  standing  in  relief. 
These  differences  of  level  make  it  possible  to  distinguish  the  constituents  under  the 
microscope  without  further  treatment.  It  is  evident  that  only  those  samples  which 
are  made  up  of  constituents  differing  much  in  hardness  can  be  so  treated.  The  free 
cementite  of  hyper-eutectoid  steel  or  of  white  cast  iron,  for  instance,  can  be  made  to 
stand  strongly  in  relief  because  it  is  so  much  harder  than  the  accompanying  pearlite 
or  other  constituents. 

Osmond  polished  his  samples  on  a  damp  piece  of  parchment  stretched  over  a  piece 
of  well-planed  wood,  and  rouge  was  rubbed  strongly  on  the  parchment.  The  block 
was  then  put  under  the  tap  and  washed  so  that  only  those  particles  of  rouge  that 
had  found  their  way  into  the  pores  of  the  parchment  were  retained.  To  distinguish 
between  raised  portions  and  cavities  the  luminous  rays  are  strongly  diaphragmed 
and  the  objective  placed  a  little  below  the  focusing  point,  is  slowly  raised.  The 
reliefs,  which  at  first  appear  brilliant  and  yellowish  on  a  relatively  darker  ground, 
gradually  become  dark  on  a  bright  ground;  the  cavities  present  inverse  appearances 
so  perfectly  that  two  photographs  of  the  same  preparation,  taken  one  a  little  below 
and  the  other  a  little  above  the  mean  focusing  point,  are  almost  positive  and  nega- 
tive to  one  another. 


62  CHAPTER  III  —  APPARATUS  AND   MANIPULATIONS 

Polish-Attack.  —  For  many  years  Osmond  obtained  his  best  preparations  by  a 
combined  polishing  and  etching  method  (polissage-attaque)  consisting  in  rubbing  the 
polished  sample  upon  a  piece  of  parchment  covered  with  some  aqueous  extract  of 
liquorice  root,  with  the  addition  of  precipitated  calcium  sulphate.  In  1899  Osmond 
and  Cartaud  recommended  replacing  the  extract  of  liquorice  by  a  diluted  solution  of 
nitrate  of  ammonium  (2  parts  by  weight  of  the  crystallized  salt  to  100  parts  of  water). 
A  piece  of  parchment  spread  tightly  over  a  smooth  board  is  soaked  with  the  solu- 
tion and  the  specimen  rubbed  upon  it  until  sufficiently  etched.  It  is  not  necessary 
to  add  any  sulphate  of  calcium. 

Etching.  —  Sorby  etched  his  specimens  with  very  dilute  solutions  of  nitric  acid  in 
water  and  this  reagent  was  widely  used  for  many  years  by  other  metallographists. 
The  water  has  now  been  replaced  by  absolute  alcohol  (Chapter  II).  Le  Chatelier 
has  mentioned  the  use  of  glycerine  as  a  satisfactory  non-oxidizing  vehicle  for  nitric 
as  well  as  for  picric  and  hydrochloric  acid. 

Osmond,  the  author  believes,  was  the  first  to  use  tincture  of  iodine.  This  tinc- 
ture is  applied  in  the  proportion  of  one  drop  per  square  centimeter  of  surface  and 
allowed  to  act  until  it  is  decolorized,  the  treatment  being  repeated  after  examina- 
tion if  needed.  Le  Chatelier  recommends  applying  the  tincture  with  the  tip  of  the 
finger  and  gently  rubbing  the  specimen. 

Stead  uses  a  solution  made  up  of  1.25  grains  of  iodine,  1.25  grains  of  iodide  of 
potassium,  1.25  grains  of  water  and  alcohol  to  make  up  100  c.  c.  After  the  iodine  has 
lost  its  color  the  sample  should  be  washed  in  water,  then  in  alcohol,  and  finally  dried 
in  a  blast  of  hot  air. 

Martens  and  Heyn  in  1904  recommended  the  use  (1)  of  an  etching  solution  con- 
taining 1  part  of  hydrochloric  acid  (1.19  sp..  gr.)  and  100  parts  of  absolute  alcohol 
and  (2)  of  1  part  of  hydrochloric  acid  in  500  parts  of  water  with  the  assistance  of 
the  electric  current. 

Heyn  used  a  solution  of  double  chloride  of  copper  and  ammonium  containing  12 
grains  of  the  salt  and  100  grains  of  distilled  water. 

To  distinguish  with  certainty  between  iron  phosphide  and  cementite,  Matweieff 
recommends  neutral  sodium  picrate  washed  several  times  with  distilled  water  to 
eliminate  the  excess  of  picric  acid  or  of  sodium  that  might  be  present.  The  sam- 
ple is  immersed  in  the  boiling  solution  for  20  minutes,  a  treatment  by  which  the 
iron  phosphide  is  strongly  attacked  while  the  cementite  and  pearlite  remain  un- 
affected. 

For  etching  austenite  and  martensite  Robin  recommends  the  use  of  a  saturated 
solution  of  picric  acid  in  alcohol,  an  immersion  of  30  seconds  to  1  minute,  wash- 
ing with  water  without  touching  the  specimen  and  drying  by  air  blast  or  simply  in 
air.  Films  of  various  tints  are  formed,  ferrite  remaining  uncolored. 

Le  Chatelier  has  used  bitartrate  of  potassium  as  an  etching  reagent.  It  leaves 
cementite  and  pearlite  uncolored,  while  it  imparts  a  dirty  coloration  to  ferrite. 

The  same  author  has  described  the  use  of  a  freshly  prepared  reagent  made  up  of 
equal  parts  of  a  solution  containing  50  per  cent  of  soda  and  of  a  solution  containing 
10  per  cent  of  lead  nitrate.  Cementite  is  quickly  colored  by  it  while  the  phosphides 
and  especially  the  silicides  are  also  attacked.  The  reagent  is  recommended  for  highly 
carburized  metals.  Medium  high  carbon  steels  of  great  purity  are  not  affected  by 
this  solution,  but  when  impure,  the  pearlite  is  energetically  acted  upon,  probably 
because  of  the  presence  of  impurity  in  that  constituent. 


CHAPTER   III— APPARATUS   AND   MANIPULATIONS  63 

Le  Chatelier  has  also  mentioned  the  use  of  a  solution  of  10  per  cent  gaseous 
hydrochloric  acid  in  absolute  alcohol  to  which  is  added  5  per  cent  of  cupric  chloride  for 
annealed  steels  and  one  per  cent  of  the  same  salt  for  hardened  steels.  Ferrite  and 
cementite  are  not  colored,  martensite  very  little,  austenite  a  little  more,  troostite 
and  sorbite  decidedly. 

The  etching  reagents  usually  applied  to  bring  out  the  structure  of  unhardened 
steel,  namely,  picric  acid,  nitric  acid,  tincture  of  iodine,  etc.,  do  not  always  yield 
satisfactory  results  in  the  case  of  hardened  steel.  Kourbatoff  discovered  a  complex 
reagent  which  often  produces  greater  contrasts  between  the  variousTconstituents.  It 
is  made  up  by  mixing  1  part  of  amyl  alcohol,  1  part  of  ethyl  alcohol,  1  part  of  methyl 
alcohol,  and  1  part  of  a  4  per  cent  solution  of  .nitric  acid  in  acetic  anhydride  and 
should  be  prepared  just  before  use. 

Heyn  recommends  for  etching  hardened  steel  a  solution  containing  one  part  of 
hydrochloric  acid  and  99  parts  of  absolute  alcohol.  More  uniform  results  are  ob- 
tained if  a  weak  current  of  electricity  be  passed  through  the  solution,  the  samples  to 
be  etched  forming  the  positive  pole  while  the  negative  electrode  may  consist  con- 
veniently of  a  piece  of  sheet  lead.  With  the  assistance  of  the  electric  current  the  use 
of  a  very  dilute  aqueous  solution  is  advisable,  namely,  1  part  of  hydrochloric  acid 
in  500  parts  of  distilled  water. 

Osmond,  likewise,  used  successfully  a  solution  of  10  per  cent  of  hydrochloric  acid 
in  water  by  which  the  martensite  is  colored  darker  than  austenite,  the  treatment  re- 
quiring several  minutes.  Osmond  writes:  "There  is  more  regularity  obtained  by 
having  the  specimen  connected,  by  means  of  a  platinum  wire,  with  the  positive  pole 
of  a  bi-chromate  cell,  a  strip  of  platinum  placed  in  the  acid  being  connected  with  the 
negative  pole.  In  this  way  the  specimen  becomes  the  anode,  and  the  platinum  the 
cathode." 

Benedicks  recommends  for  the  etching  of  martensito-austenitic  steel  a  5  per  cent 
alcoholic  solution  of  metanitrobenzol-sulphonic  acid  which  always  darkens  martensite 
more  than  austenite.  Immersions  of  some  15  seconds  are  generally  sufficient. 

Hilpert  and  Colver-Glauert  have  described  the  use  of  sulphurous  acid  for  non- 
pearlitic  steels  and  for  pig  iron.  A  saturated  solution  of  sulphur  dioxide  in  water  is 
prepared  and  3  or  4  per  cent  of  that  solution  in  water  used.  The  time  of  etching 
varies  between  7  seconds  and  1  minute.  Alcohol  may  be  substituted  for  water 
in  which  case  the  etching  lasts  several  minutes.  The  treatment  causes  the  deposi- 
tion of  layers  of  iron  sulphide  of  different  thickness  and,  therefore,  of  different  colors, 
on  the  various  constituents. 

Rosenhain  and  Haughton  report  interesting  results  obtained  with  a  solution  made 
up  as  follows: 

Ferric  chloride  30  grams,  hydrochloric  acid  (cone.)  100  cubic  centimeters,  cupric 
chloride  1.0  gram,  stannic  chloride  0.5  gram,  water  1  liter. 

The  polished  specimen  which  should  be  absolutely  clean  is  immersed  in  the  solu- 
tion for  10  seconds  to  2  minutes  according  to  the  character  of  the  metal.  The  action 
of  the  reagent  consists  in  the  deposition  on  the  surface  of  the  steel  of  thin  films  of 
metallic  copper  which  vary  in  thickness  and  therefore  in  appearance  with  the  various 
constituents  present,  producing  an  effect  not  unlike  that  obtained  with  heat  tinting. 
The  authors  write  that  "in  practically  pure  carbon  steels  the  new  reagent  yields  a 
pattern  which  is  the  reverse  of  that  obtained  with  the  ordinary  etching  reagents, 
such  as  picric  acid,  ferrite  being  darkened  while  pearlite  remains  white;  on  hardened 


64  CHAPTER  III  —  APPARATUS  AND   MANIPULATIONS 

steels,  especially  those  of  low  and  moderate  carbon  contents,  the  reagent  yields  very 
clear  results  particularly  in  the  etching  of  martensite,  although  in  this  case  the  pat- 
terns are  not  reversed  as  compared  with  those  obtained  with  picric  acid,  sulphur 
dioxide,  etc.  The  principal  interest  and  importance  of  the  new  reagent,  however, 
lies  in  the  fact  that  it  reveals  in  a  clear  and  striking  manner  the  distribution  of  phos- 
phorus, particularly  in  mild  steels." 

Howe,  according  to  0.  F.  Hudson,  uses  in  his  laboratory  Heyn's  reagent  (10 
per  cent  copper  ammonium  chloride  in  water)  for  low  carbon  iron  and  steel.  He 
finds  that  it  "shows  grain  boundaries  readily,  rather  obscures  Neumann  lines"  and 
that  it  is  the  best  reagent  for  etching  figures  and  pits.  Hoyt  uses  a  similar  solu- 
tion "principally  for  microscopic  examination  of  low  carbon  steel,  wrought  iron, 
white  cast  iron,  and  in  the  determination  of  phosphorus  segregation  in  wrought 
iron." 

Electrolytic  Etching.  • —  Le  Chatelier  was  one  of  the  first  to  advocate  the  use  of 
the  electric  current  in  order  to  obtain  a  more  uniform  action  in  etching  iron  and  steel 
samples.  Sheet  lead  may  be  used  for  the  positive  electrode  and,  as  electrolyte,  a 
10  per  cent  solution  of  chloride  or  sulphate  of  ammonium  gives  good  results.  The 
current  needed  varies  between  0.001  and  0.01  amperes  per  square  centimeter. 


Fig.  58.  —  Arrangement  for  electrolytic  etching. 

Electrolytic  etching  has  been  described  by  Cavalier  (1909).  A  few  cubic  centi- 
meters of  the  electrolyte  are  placed  in  a  platinum  dish  C  (Fig.  58)  connected  with 
one  pole  of  the  battery  P;  the  specimen  E  connected  with  the  other  pole  is  placed 
in  the  solution,  a  piece  of  filter  paper  A  being  inserted  between  the  dish  and  the 
polished  surface  of  the  specimen.  The  current  is  regulated  through  the  rheostat  R. 
Four  or  five  volts  are  required  with  an  intensity  of  0.001  to  0.01  amperes  per  square 
centimeter.  The  attack  lasts  from  a  few  seconds  to  a  few  minutes. 

Hot  Etching.  —  Steel  while  at  a  high  temperature  (red  heat)  has  been  etched 

(1)  by  Saniter  in  molten  calcium  chloride  heated  to  the  desired  temperature,  and 

(2)  by  Baykoff  in  a  current  of  gaseous  hydrochloric  acid. 

Washing  and  Drying.  —  After  removing  the  specimens  from  the  etching  bath,  the 
author  washes  them  in  alcohol  and  dries  them  in  an  air  blast.  Washing  in  water,  in 
caustic  potash,  in  lime  water,  and  in  ether  has  also  been  recommended  as  well  as 
the  use  of  fine  linen  cloth  and  of  a  hot  blast  for  drying. 

Preserving.  —  The  author  preserves  his  etched  specimens  in  desiccators  and  in 
air-tight  cabinets. 

Several  protective  coatings  have  been  described.  Stead  covers  them  with  paraf- 
fin wax  dissolved  in  benzole,  which  is  removed  by  wiping  with  a  clean  linen  rag  mois- 
tened with  benzole,  when  it  is  desired  to  examine  the  specimens.  Le  Chatelier  applies 
a  coating  of  "zapon,"  a  solution  of  gun  cotton  in  amyl  acetate  sufficiently  transparent 
to  allow  examination  with  the  highest  powers. 


CHAPTER   III  —  APPARATUS   AND   MANIPULATIONS 


65 


MOUNTING   AND   MOUNTING   DEVICES 

The  author's  special  holders  for  placing  the  prepared  samples  on  the  stage  of  the 
microscope  have  been  described  in  Chapter  I.  Other  methods  have  been  used  and 
are  still  employed  by  some  workers,  namely,  (1)  mounting  in  some  plastic  material 
and  (2)  the  use  of  leveling  stages. 

Plastic  Mounting.  • —  Osmond  mounted  his  specimens  by  embedding  them  in  a 
little  soft  wax  placed  upon  a  glass  plate.  The  leveling  is  managcd-by  means  of  two 
pieces  of  glass  tube  of  equal  height,  one  on  each  side  of  the  sample. 

Stead  places  the  specimens  polished  face  down  on  a  piece  of  plate  glass  (Fig.  59) 
and  surrounds  them  with  brass  cylinders  accurately  turned.  A  piece  of  plastic  wax 
is  stuck  upon  the  center  of  a  glass  microscope  slide  and  is  then  pressed  upon  the  sec- 


,.  Glass  slide 


Brass 


vkSS^/i™  • -«rtj/«?/>»^ 

-  'Specimen 


Fig.  59.  —  Stead's  mounting  device. 
(C.  H.  Desch's  Metallography.) 


Fig.  60.  —  Gulliver's  mounting  device. 


tion  till  the  glass  slide  comes  in  contact  with  the  brass  ring.  The  specimen  adheres 
to  the  wax  and  the  mounting  is  complete. 

Gulliver  (1908)  describes  the  device  (Fig.  60)  for  mounting  specimens.  It  consists 
of  a  circular  ring  faced  on  its  upper  surface  A,  and  screwed  internally  at  B  to  fit  the 
foot,  of  which  the  upper  end  C  is  also  faced.  The  distance  between  the  parallel  faces 
A  and  C  can  thus  be  adjusted.  The  specimen  is  placed  at  D  and  a  glass  slide  E  with 
some  soft  modeling  clay  or  wax  is  pressed  upon  it  until  the  glass  touches  the  ring 
at  AA. 

Mechanical  mounting  devices  working  on  the  principle  of  the  microtome  have 
also  been  used.  They  have  been  described  by  M.  A.  Richards:  "Projecting  from  a 
cylindrical  metal  base  three  inches  in  diameter,  is  a  threaded  upright  three  and  one- 
half  inches  in  diameter.  A  cylindrical  nut  or  collar  three  inches  high  and  two  and 
one-half  inches  outside  diameter  screws  on  the  threaded  upright.  A  small  circle  of 
chamois  skin  is  placed  on  the  top  of  the  thread  upright  to  protect  the  etched  face  of 
the  micro-section.  To  mount  a  section,  place  it  face  down  on  the  chamois  skin,  press 
upon  the  upper  projecting  portion  a  lump  of  beeswax,  and  upon  this  place  the  ground 
glass  (ground  surface  down).  A  few  revolutions  of  the  collar  will  cause  the  glass  to 
rest  upon  the  upper  edge  of  the  collar,  and  the  adhesion  of  the  glass  and  beeswax  to 
the  specimen  may  be  made  complete  by  slowly  turning  the  collar  down  with  one  hand 


66 


CHAPTER   III  —  APPARATUS   AND   MANIPULATIONS 


while  keeping  the  glass  base  in  close  contact  with  the  collar-top  with  the  other  hand. 
In  this  manner,  no  matter  how  irregular  the  section,  the  parallelism  of  the  etched 
surface  and  the  glass  base  may  be  very  quickly  and  accurately  obtained." 

The  mounting  device  (Fig.  61)  is  constructed  by  Watson  and  Sons.    It  consists  of 
two  horizontal  plates,  the  upper  one  being  capable  of  vertical  movement  but  always 


Fig.  61.  —  Watson  and  Sons'  mounting 
device. 


Fig.  62.  — •  Watson  and  Sons' 
leveling  stage. 


remaining  parallel  to  the  lower  one.  The  specimen  is  placed  with  its  polished  surface 
on  the  lower  plate,  and  the  upper  plate  carrying  a  glass  slip  to  which  some  suitable 
clay  or  wax  is  attached  is  lowered  into  contact. 

Leveling  Stages.  —  The  leveling  stage  (Fig.  62)  is  one  of  several  constructed  by 
Watson  and  Sons,  London.  The  specimen  is  held  by  two  rotating  jaws  and  can  be 
leveled  by  means  of  the  screws  A  and  B  BI. 


Fig.  63.  —  Huntington's  leveling  stage. 


Professor  A.  K.  Huntington  devised  the  leveling  stage  shown  in  Figure  63.  It  is 
provided  with  a  ball  and  socket  joint  for  leveling,  permitting  the  placing  of  the 
sample  in  any  position. 

Other  forms  of  leveling  stages  are  shown  in  some  of  the  illustrations  in  the  follow- 
ing pages  as  part  of  some  metallurgical  microscopes. 


CHAPTER   III  —  APPARATUS   AND   MANIPULATIONS 


67 


M  KTALLURGICAL  MICROSCOPES 

The  microscopes  and  accessories  used  by  the  author  have  been  fully  described. 
In  the  following  pages  instruments  used  by  some  other  workers  or  described  by  them, 
as  well  as  those  manufactured  by  well-known  makers,  are  mentioned. 


Fig.  04.  —  Le  Chatelior's  invert  ed  metallurgical  microscope. 
Early  form. 


Fig.  65.  —  Le  Chatelier's  inverted  metallurgical  microscope. 


Le  Chatelier.  —  In  1897  Le  Chatelier  devised  an  inverted  microscope  which  later 
he  greatly  improved  and  which  is  now  constructed  with  unimportant  modifications 
by  several  microscope  makers.  An  early  form  of  Le  Chatelier's  instrument  is  shown 
in  Figure  64  and  its  more  recent  construction  in  Figures  65  and  66.  The  objective 


68 


CHAPTER  III  —  APPARATUS  AND   MANIPULATIONS 


0) 

1 

6 


CHAPTER  III  —  APPARATUS  AND  MANIPULATIONS 


69 


B  (Fig.  65)  is  directed  upwards  while  the  eye-piece  0,  placed  horizontally,  receives 
the  image  by  the  reflection  of  a  totally  reflecting  prism  F  placed  below  the  objective. 
The  prism  F  may  be  rotated  by  means  of  the  milled  head  P  and  the  light  reflected  by 
the  objective  turned  at  will  into  the  tube  G  and  the  eye-piece  0  for  visual  examination 


Fig.  67.  —  Le  Chatelier's  inverted  metallurgical  microscope. 

or  into  another  tube  connected  with  a  camera  for  photographing  (Fig.  66).  The 
light  is  condensed  by  the  lens  A  and,  being  deflected  at  right  angles  by  the  prism  J, 
passes  through  the  objective  B  and  reaches  the  object  M  placed  on  the  stage  E.  In 


Fig.  68.  —  Device  for  placing 
specimens  on  the  stage  of 
the  Le  Chatelier  micro- 
scope in  a  fixed  position. 
(Le  Grix.) 


case  the  light  is  placed  at  a  higher  level  than  the  condensing  lens  A,  it  must  be  re- 
ceived by  a  totally  reflecting  prism  H  which  directs  it  into  the  condenser  A.  D  is  a 
diaphragm  placed  at  the  principal  focus  of  the  complex  optical  system  composed  of 
the  objective  B,  the  illuminating  prism  J,  and  the  lens  A.  The  opening  as  well  as  the 


70 


CHAPTER   III  —  APPARATUS   AND   MANIPULATIONS 


position  of  the  diaphragm  may  be  altered.  Another  diaphragm  placed  at  I  affords 
a  means  of  stopping  the  light  which  would  fall  upon  parts  of  the  preparation  outside 
of  the  portion  examined  and  which  would  increase  the  blur  resulting  from  the  reflec- 
tion of  useless  rays  by  the  back  lenses  of  the  objective.  In  the  early  construction  of« 


Fig.  69.  —  Inverted  metallurgical  microscope  constructed  by  E.  Leitz. 


Fig.  70.  —  Inverted  metallurgical  microscope  constructed  by  E.  Leitz. 


this  instrument  when  the  object  was  to  be  photographed  the  prism  F  was  withdrawn 
from  the  path  of  light  and  the  image  allowed  to  form  on  a  photographic  plate  placed 
below  (Fig.  64). 

A  slightly  different  construction  is  shown  in  Figure  67.     For  photographic  pur- 
poses the  image  forms  on  a  plate  placed  in  a  holder  rigidly  connected  with  the  instru- 


CHAPTER   III  —  APPARATUS   AND   MANIPULATIONS 


71 


ment,  no  eye-piece  being  used.  As  the  distance  between  the  photographic  plate  and 
the  objective  is  short,  very  small  photomicrographs  are  obtained,  which  must  gen- 
erally be  subsequently  enlarged.  Z  is  a  plate  carrying  an  eye-piece  for  use  with  the 
long  bellows  camera  (Fig.  66).  The  Le  Chatelier  microscopes  are  constructed  by 
Ph.  Pellin  of  Paris. 

In  order  to  be  able  to  examine  identical  portions  of  the  same  specimen  at  different 
times  with  the  Le  Chatelier  microscope,  Le  Grix  (1907)  suggested  the  arrangement 
shown  in  Figure  68.  A  circular  metallic  disk  with  rectangular  opening  RR'  and  carry- 
ing two  pointed  stops  A  and  B  is  fitted  to  the  stage.  A  file  mark  E  is  made  in  the 
specimen  .17,  which  is  then  placed  on  the  stage  so  that  the  stop  A  enters  the  groove  E 


Fig.     71.  —  Metallurgical     microscope  con- 
structed by  E.  Leitz. 


Fig.  72.  —  Metallurgical  microscope 
constructed  by  E.  Leitz. 


while  the  specimen  presses  against  the  other  stop  B,  in  this  way  securing  a  constant 
position  for  the  object. 

Ernst  Leitz.  —  A  slightly  modified  form  (Fig.  69)  of  the  Le  Chatelier  inverted 
microscope  is  made  by  Ernst  Leitz  of  Wetzlar,  Germany.  The  modifications  were 
suggested  by  Guertler.  The  stage  and  illuminating  appliances  are  shown  on  a  larger 
scale  in  Figure  70. 

The  same  maker  also  manufactures  the  microscope  shown  in  Figures  71  and  72 
designed  by  W.  Campbell.  The  stage  can  be  removed  and  the  upper  part  of  the 
instrument  attached  to  the  base  for  the  examination  of  large  surfaces. 

P.  F.  Di/jiinll/i.  —  P.  F.  Dujarclin  and  Co.  of  Diisseldorf  construct  a  Le  Chatelier 
inverted  microscope  as  shown  in  Figure  73.  They  also  make  the  microscope  (Fig.  74) 
in  which  the  vertical  illuminator  carries  its  own  source  of  light  and  condenser. 

C.  Reichert.  —  The  metallurgical  microscope  (Fig.  75)  designed  by  Professor 
Rejto  is  made  by  C.  Reichert  of  Vienna.  The  position  of  the  vertical  illuminator 
immediately  below  the  eye-piece  should  be  noted.  The  stage  is  provided  with  a  level- 
ing mechanism.  The  same  maker  manufactures  an  inverted  Le  Chatelier  microscope 
as  shown  in  Figure  76. 

By  means  of  a  mirror  Sp  (Fig.  77)  placed  in  the  camera  box  it  is  possible  to  focus 
the  image  on  a  ground  glass  M,  on  the  same  side  of  the  instrument  as  the  observer, 


72 


CHAPTER   III  —  APPARATUS   AND   MANIPULATIONS 


o 
O 


Q 

fa 


01 

p. 


f 


0> 

s 


I 

c 


CHAPTER   III  —  APPARATUS   AND   MANIPULATIONS 


73 


thereby  facilitating  the  manipulations.  During  exposure  the  mirror  which  is  hinged 
is  folded  close  to  the  side  M  permitting  the  image  to  form  on  the  photographic  plate 
at  K. 

Robin.  • —  The  microscope  and  photographic  attachment  shown  in  Figure  78  was 
designed  by  Robin.    Visual  examination  is  possible  only  on  the  screen  of  the  camera. 


Oc 


Fig.  74.  —  Metallurgical  microscope  constructed 
by  P.  F.  Dujarclin  and  Co. 


Fig  75.  —  Metallurgical  microscope  constructed 
by  C.  Reichert. 


The  stage  consisting  of  a  smooth  disk  is  tilting  and  the  specimen  is  fastened  upon  it 
with  wax.  To  secure  an  accurately  horizontal  position  of  the  polished  surface,  a 
plug  with  a  perfectly  flat  surface  is  screwed  into  the  microscope  in  place  of  the  ob- 
jective and  the  stage  raised  until  the  specimen  coming  in  contact  with  the  plug,  the 
latter  through  gentle  pressure  causes  the  polished  surface  to  assume  a  horizontal 
position.  The  plug  is  then  removed  and  the  objective  inserted. 

Scientific  Materials  Co.  —  The  inverted  microscope  and  photographic  attachment 
(Fig.  79)  are  made  by  the  Scientific  Materials  Co. 


74 


CHAPTER  III  — APPARATUS  AND   MANIPULATIONS 


Fig.  76.  —  Inverted  Metallurgical  microscope.     (C.  Reichert.) 


C.REIOHERT 


I 


M 


Fig.  78.  —  Metallurgical  microscope  designed  by  Robin. 


CHAPTER   III  — APPARATUS   AND   MANIPULATIONS 


75 


Martens.  —  The  Martens  metallurgical  microscope  (1899)  made  by  Zeiss  of  Jena 
is  shown  in  Figure  80.  It  can  be  used  horizontally  only,  the  tube  is  very  wide  and 
the  vertical,  mechanical  stage  is  provided  with  both  coarse  and  fine  adjustments  Y 
and  Z  and  with  leveling-screws  act.  The  flexible  connection  /  permits  the  focusing 


6 

.3 
a 


<a 

a 
j 

1 


of  the  object  from  the  camera  screen.     The  instrument  is  designed  especially  for 
photography. 

A  complete  Zeiss  equipment  including  a  large  electric  arc  lamp  is  shown  in 
Figure  81.  It  will  be  noted  that  the  mounting  of  the  camera  is  entirely  separate 
from  that  of  the  other  parts. 


76 


CHAPTER  III  —  APPARATUS  AND   MANIPULATIONS 


Fig.  80.  —  Martens  metallurgical  microscope. 


Fig.  81.  —  Martens-Zeiss  metallurgical  microscope  and  camera. 


CHAPTER   III  —  APPARATUS   AND   MANIPULATIONS 


77 


Rosenhain.  —  The  microscope  shown  in  Figure  82  was  constructed  by  R.  and  J. 
Beck  for  Rosenhain.  The  stage  is  mechanical  and  provided  with  coarse  and  fine 
adjustments,  and  all  controlling  heads  are  placed  beneath.  Appliances  are  provided 


JiHi 


Fig.  82.  —  Rosenhain  metallurgical  microscope. 


for  various  kinds  of  illumination.  The  necessary  alteration  of  focus  for  photograph- 
ing can  be  done  at  the  eye-piece  by  a  suitable  arrangement  provided  for  that  purpose. 
Osmond.  —  Osmond  used  a  Nachet  microscope  of  the  ordinary  type  connected 
with  a  vertical  camera  and  a  prism  illuminator.  He  writes,  however,  that  special 
metallurgical  microscopes  "are  certainly  to  be  preferred."  In  Osmond's  opinion  the 
vertical  is  very  much  superior  to  the  horizontal  camera  for  studying  metals. 


78 


CHAPTER   III  —  APPARATUS   AND    MANIPULATIONS 


Fig.  83.  —  Nachet  metallurgical  microscope. 


Fig.  85.  —  Cornu-Chaipy  metallurgical 
microscope. 


Fig.  84.  —  Guillemin-Nachet 
prism  illuminator. 


CHAPTER   III  — APPARATUS   AND   MANIPULATIONS 


79 


Nachet. —  Nachet  of  Paris  constructs  the  metallurgical  microscope  (Fig.  83). 
The  vertical  illuminator  carries  a  tube  provided  with  an  iris  diaphragm.  The  stand 
is  to  be  used  in  the  vertical  position  only.  The  stage  has  a  coarse  vertical  adjust- 
ment. A  similar  microscope  is  made  with  mechanical  stage  provided  with  both 
coarse  and  fine  adjustments. 

The  prism  illuminator  (Fig.  84)  designed  by  Guillemin  is  made  by  Nachet.  A 
lateral  as  well  as  a  slight  tilting  motion  may  be  imparted  to  the  prism  through  the 
milled  heads  H  and  C. 

Nachet 's  illuminating  objectives  have  been  described  and  illustrated  in  Chapter  I. 


Fig.  80.  —  Metallurgical  microscope  constructed  by 
Wul  son  and  Sons. 


Cornu-Charpy.  —  The  arrangement  shown  in  Figure  85  was  used  by  Charpy.  The 
vertical  illuminator  G  consists  of  four  thin  glass  plates  placed  at  an  angle  of  45° 
immediately  below  the  eye-piece  and  it  receives  the  light  reflected  by  the  totally 
reflecting  prism  P.  This  prism  is  so  mounted  that  it  can  rotate  freely  around  the 
axis  of  the  microscope  and  also  around  the  axis  GP  of  the  tube  to  which  it  is  at- 
tached, thus  making  it  possible  to  receive  upon  it  the  light  proceeding  from  a  source 
of  light  placed  anywhere. 

Watson  and  Sons.  —  The  metallurgical  microscope  (Fig.  86)  was  constructed  in 
1904  by  Watson  and  Sons  of  London.  The  stage  is  provided  with  both  coarse  and 


80 


CHAPTER   III  —  APPARATUS   AND    MANIPULATIONS 


Fig.  87.  —  Horizontal  metallurgical  microscope  constructed  by 
Watson  and  Sons. 


Fig.  88.  —  Stead's  workshop  microscope. 
(J.  Swift  and  Son.) 


Fig.  89. 


Fig.  90. 


Ufl 


CHAPTER  III  — APPARATUS  AND   MANIPULATIONS 


81 


fine  adjustments.  The  same  makers  following  Martens  construct  the  horizontal 
metallurgical  microscope  (Fig.  87)  and  also  simpler  and  less  expensive  models  of 
metallurgical  microscopes,  some  of  them  provided  with  leveling  stages. 

Workshop  Microscopes.  —  Metallurgical  microscopes  of  simple  construction,  stage- 
less,  easily  transportable  and  especially  adapted  to  the  examination  of  polished  spots 


Fig.  91.  —  Workshop  microscope.    (W.  Watson  and  Sons.) 

on  large  pieces,  in  the  mills  or  shops,  are  known  as  workshop  microscopes.  The  model 
Figure  88  is  constructed  by  J.  Swift  and  Son  according  to  Stead's  designing.  For  very 
low-power  work  the  lamp  and  vertical  illuminator  are  placed  below  the  objective  as 
shown  in  Figure  89,  while  for  higher  power  they  are  placed  above  (Fig.  90).  A  sim- 


Fig.  92.  —  Illuminating  attachment  for  workshop  microscope.     (W.  Watson  and  Sons.) 


ilar  instrument  (Fig.  91)  is  manufactured  by  W.  Watson  and  Sons.  The  illuminating 
attachment  is  shown  in  Figure  92.  The  same  illuminator  may  be  used  with  any 
microscope  for  vertically  illuminating  opaque  objects. 

Workshop  microscopes  constructed  by  C.  Eeichert  according  to  designs  of  Martens, 
Rejto,  and  Jiiptner  are  shown  respectively  in  Figures  93,  94,  and  95. 


82 


CHAPTER   III  —  APPARATUS   AND   MANIPULATIONS 


Fig.  93.  —  Martens'  workshop  microscope.    (C.  Reichert.) 


Fig.  94.  —  Retjo's  workshop  microscope.     (C.  Rrichert.) 


CHAPTER   III  — APPARATUS   AND    MANIPULATIONS 


83 


Fig.  95.  —  Jiiptiu'r's  workshop  Tiiicroscope.     (C.  Reichert.) 


Fig.  96.  —  Tassin's  workshop  microscope.     (Bausch  and  Lomb  Optical  Co.) 


84 


CHAPTER  III  —  APPARATUS  AND   MANIPULATIONS 


The  Tassin  Workshop  Microscope,  manufactured  by  the  Bausch  and  Lomb 
Optical  Co.,  is  shown  in  Figure  96,  with  camera  attached. 

R.  and  J.  Beck.  • —  In  1904  R.  and  J.  Beck  of  London  constructed  the  prism  ver- 
tical illuminator  shown  in  Figure  97.  The  device  is  fitted  with  an  iris  diaphragm 
beneath  the  prism  for  cutting  off  outside  light,  and  a  plate  of  stops  so  arranged  that 
the  position  of  the  beam  of  light  impinging  on  the  prism  can  be  varied  until  parallel 
light  of  the  right  angle  is  obtained.  The  same  makers  construct  the  instrument 


Fig.  97.  —  Beck  prism 
illuminator. 


Fig.  98.  —  Beck  surface  microscope. 


shown  in  Figure  98  for  the  examination  of  large  metallic  surfaces.    The  Rosenhain 
microscope  described  in  these  pages  is  likewise  made  by  R.  and  J.  Beck. 

The  vertical  illuminator,  Figure  99,  is  manufactured  by  the  Bausch  and  Lomb 
Optical  Co.  It  has  two  small,  square  mirrors,  one  for  high  power  objectives,  the 
other  for  low  powers.  The  aperture  in  the  side  of  the  mounting  is  fitted  with  a  small 
tube  carrying  a  condensing  adjustable  lens,  for  focusing  the  light.  The  mirror 
carrier  is  mounted  on  a  slide  providing  for  oblique  as  well  as  central  illumination. 


Fig.  99.  —  Vertical  illumi- 
nator with  condensing  at- 
tachment. (Bausch  and 
Lomb  Optical  Co.) 


F .  Koristka.  —  The  prism  vertical  illuminator  (Fig.  100)  was  described  by 
F.  Koristka  of  Milan  in  1905.  An  iris  diaphragm  placed  in  front  of  the  prism  controls 
the  light  which  it  receives.  By  pulling  out  the  arm  carrying  the  prism  the  latter  may 
be  removed  from  the  field. 

Ph.  Pellin.  —  The  Le  Chatelier  inverted  microscope  is  constructed  by  Ph.  Pellin 
of  Paris.  The  same  makers  also  manufacture  a  portable  microscopic  outfit  designed 
by  Guillet  (Trousse  de  Metallographie).  It  includes  a  small  electric  motor  for  polish- 
ing, a  vertical  microscope  so  constructed  that  it  can  be  fastened  to  any  object  it  is 


CHAPTER  III  —  APPARATUS  AND   MANIPULATIONS 


85 


desired  to  examine,  files,  emery-papers,  etching  reagents,  etc.    All  parts  are  com- 
pactly placed  in  a  carrying  case. 

Carl  Zeiss.  —  The  instruments  used  by  Martens  and  Heyn  already  described  in 
these  pages  are  constructed  by  Carl  Zeiss  of  Jena.  The  prism  vertical  illuminator 
made  by  the  same  firm  has  been  described  and  illustrated  in  Chapter  I. 


Fig.  100.  —  Koristka  prism  illuminator. 


Spencer  Lens  Co.  —  The  Spencer  Lens  Co.  of  Buffalo,  N.  Y.,  manufacture  a 
vertical  metallurgical  microscope  with  movable  stage. 

Bausch  and  Lomb  Optical  Co.  • —  The  microscopes  and  accessories  used  and  de- 
signed by  the  author  and  fully  described  in  Chapter  I  are  manufactured  by  the  Bausch 
and  Lomb  Optical  Co.  of  Rochester,  N.  Y. 


CHAPTER  IV 

PURE  METALS 

Microstructure.  —  When  a  properly  prepared  sample  of  a  pure  metal  is  examined 
under  the  microscope,  the  revealed  structure  generally  presents  the  appearance  of  a 
polygonal1  network  (Figs.  101  and  102),  an  indication  that  the  metal  itself  is  composed 
of  irregular  polyhedral2  grains,  each  mesh  or  polygon  of  the  network  representing  a 
section  through  a  polyhedron.2 

Crystallization.  —  When  a  substance  passes  from  the  liquid  to  the  solid  state,  the 
process  of  solidification  is  generally  accompanied  by  crystallization,  i.e.  the  molecules 


Fig.   101.  —  Pure  Gold.     Cast.     Magnified  Fig.  102.  —  Pure  copper.     Magni- 

50  diameters.     (Andrews.)  fied  8  diameters.     (Houghton.) 


of  the  substance  so  arrange  themselves  as  to  form  small  solids  of  regular  geometrical 
outlines,  such  as  cubes,  octahedra,3  etc.  Each  of  these  spontaneously  formed  sym- 
metrical solids  is  called  a  crystal  and  any  substance  made  up  of  crystals  is  said  to  be 
crystalline. 

Crystals  possess  the  property  of  breaking  more  readily  in  one  or  more  directions. 
This  property  is  called  "cleavage."  The  planes  of  cleavage  or  direction  of  easy  rup- 
ture are  generally  parallel  to  the  faces  of  the  crystals.  A  cubic  crystal,  for  instance, 

1  A  polygon  is  a  closed  geometrical  figure  with  straight  sides  (necessarily  three  or  more). 

2  A  polyhedron  (plural,  polyhedra  or  polyher         1  is  a  closed  geometrical  solid  hounded  by 
plane  (smooth)  faces  (necessarily  four  or  more). 

3  An  octahedron  (plural,  octahedra  or  octaheo         j  is  a  geomet  rical  solid  (a  polyhedron)  bounded 
by  eight  plane  faces. 

86 


CHAPTER   IV  — PURK    METALS 


87 


splits  readily  in  three  planes  parallel  to  the  three  sets  of  faces  of  the  solid.  In  Figure 
103,  ABC,  DEF,  and  GHI  indicate  the  cleavage  planes  of  a  cubic  crystal.  The  di- 
rection of  its  cleavage  planes  constitutes  the  orientation  of  the  crystal. 

Solid  substances  which  are  not  crystalline  are  said  to  be  "amorphous."  They  are 
characterized  by  the  absence  of  any  symmetrical  grouping  of  their  molecules.  Glass 
is  a  good  example  of  an  amorphous  substance. 

Idiomorphic  Crystals.  —  When  the  fluidity  of  a  substance  and  other  conditions  are 
such  that  the  formation  and  growth  of  the  crystals  are  given  free  play,  perfect  (and 
sometimes  very  large)  crystals  are  produced.  These  perfect  crystals,  with  faultless 
geometrical  outlines,  perfect  cubes  for  instance,  are  called  "idiomorphic"  crystals. 

Allotrimorphic  Crystals.  —  When  the  free  development  of  crystals  is  hindered  by 
less  favorable  crystallizing  conditions,  such  for  instance  as  collision  or  contact  with 
other  crystals,  likewise  in  process  of  formation,  the  regular  external  form  is  not  pre- 
served and  the  resulting  imperfect  crystals  are  called  "  allotrimorphic  "  crystals,  also, 


Fig.  103.  —  Cleavage  planes. 
(Mellor.) 


Fig.  104.  —  Crystallization  from 
centers.     (Desch.) 


but  more  seldom,  "anhedrons"  or  faceless  crystals.  Such  crystals  are  said  to  have 
taken  their  shape  from  their  surroundings.  It  should  be  noted,  however,  that  allotri- 
morphic crystals,  like  idiomorphic  crystals,  are  composed  of  crystalline  matter.  An 
allotrimorphic  crystal  may  be  regarded  as  resulting  from  the  mutilation  of  an  idio- 
morphic crystal,  the  mutilation  affecting  the  external  shape  only,  and  not  the  crystal- 
line character  of  the  substance.1 

Crystallization  of  Metals.  —  Metals  when  they  solidify  generally  give  rise  to  the 
formation  of  allotrimorphic  crystals.  The  explanation  for  this  is  to  be  found  in  the 
fact  that  crystallization  sets  in  simultaneously  at  many  different  centers  (see  Fig.  104). 
From  each  center  a  crystal  grows  through  successive  addition  of  crystalline  matter 
similarly  oriented,  until  meeting  with  other  surrounding  crystalline  growths,  radiating 
from  other  centers,  the  free  development  of  its  external  form  is  arrested.  The 
mechanisms  of  this  crystalline  growth  has  been  clearly  illustrated  by  Rosenhain  (Fig. 
105).  Five  stages  of  the  progressive  growth  of  the  grains  are  shown  in  a  to  e,  while 
in  /  the  final  polygonal  outline  of  the  grains  is  indicated.  The  polygonal  networks 
shown  in  Figures  101  and  102,  representing  the  structure  respectively  of  pure  gold  and 
pure  copper,  do  not  indicate,  therefore,  cleavage  planes,  i.e.  outlines  of  true  crystals, 

'•  Crystalline  groups  or  aggregates  of  allot rin  'rphic  crystals  are  sometimes  called  "crystallites" 
while  if  they  assume  some  distinct  form  they  t  be  further  described  as  "dcndrites"  or  "tree- 
like," "fern  leaves,"  "star-like  crystallite's,  "en  lline  grains,"  etc. 


88 


CHAPTER  IV  — PURE   METALS 


but  merely  boundaries  or  junction  lines  between  adjacent  crystalline  growths  or 
"grains."  They  mark  the  regions  where  neighboring  crystalline  growths  collided  with 
resulting  distortion  of  their  external  forms.  Some  believe,  as  explained  later,  that  a 
little  amorphous  metal  is  left  at  the  boundaries. 


(a) 


ft) 


(d) 


\  (e)  (/) 

Fig.  105.  —  Illustrating  the  growth  of  crystalline  grains.     (Rosenhain.) 

The  number  of  crystalline  grains  of  which  a  certain  mass  of  metal  is  composed 
must  depend,  therefore,  upon  the  number  of  centers  or  nuclei  at  which  crystallization 
begins  and  this  in  turn  probably  depends  upon  the  metal  itself,  its  rate  of  cooling  and, 


CHAPTER   IV  — PURE   METALS  89 

according  to  some,  its  purity;  the  slower  its  solidification  and  the  greater  its  purity 
the  fewer  the  nuclei  and  therefore  the  larger  the  crystalline  grains. 

Grains  of  Metals.  —  If  crystallographers  were  interested  in  the  constitution  and 
structure  of  metals  they  would  undoubtedly  refer  to  the  small  polyhedral  grains  of 
which  they  are  composed  as  allotrimorphic  crystals.  This  expression,  however, 
although  the  only  scientifically  correct  one,  does  not  appeal  to  metallurgists,  who 
generally  call  these  imperfect  crystals  "crystal  grains,"  "crystalline  grains,"  or  even 
simply  "grains."  The  expression  crystalline  grains  which  is  quite  common  appears 
satisfactory  since  it  suggests  the  two  main  facts  to  be  remembered  as  to  the  nature 
of  the  grains,  (1)  that  they  are  not  perfect  crystals  and  (2)  that  they  are  nevertheless 
crystalline.  Dana  himself  writes  that  cast  iron  is  made  up  of  crystalline  grains.  He 


Fig.  106.  —  Pure  copper.  Magnified  125 
diameters.  (F.  C.  Langenberg  in  the 
author's  laboratory.) 

says  further:  "Crystallization  produces  masses  made  of  crystalline  grains  when  it 
cannot  make  distinct  crystals."  Nor  is  there  very  great  objection  if,  for  the  sake 
of  brevity,  the  word  grain  alone  be  used,  bearing  in  mind  once  for  all  the  crystalline 
character  of  metals. 

To  render  the  polygonal  boundaries  of  crystalline  grains  visible  under  the  micro- 
scope the  highly  polished  metallic  surface  must  generally  be  treated  by  an  acid  or 
some  other  chemical  reagent  capable  of  dissolving  or  corroding  the  metal,  with  or 
without  deposition  of  a  film  of  some  precipitated  matter.  According  to  Ewing  these 
boundaries  are  made  evident  by  the  differential  action  of  the  acid  which  produces 
differences  of  level  by  attacking  one  grain  more  energetically  than  its  neighbors. 
Each  of  the  short  sloping  surfaces  which  connect  one  grain  with  another  appears 
black  under  vertical  illumination  because  it  does  not  reflect  the  light  back  into  the 
tube  of  the  microscope. 

Another  explanation  of  the  nature  of  these  boundaries  is  offered  later. 


90 


CHAPTER  IV  — PURE  METALS 


Crystalline  Orientation  of  the  Grains.  —  A  slight  chemical  attack  or  etching  of 
the  polished  samples  brings  out  merely  the  polygonal  structure  described  and  illus- 
trated in  the  preceding  pages.  If  the  etching  be  somewhat  deeper,  however  (through 
the  use  of  a  stronger  reagent  or  because  of  a  longer  attack),  it  is  observed  that  the 
polygons  or  meshes  of  the  network  are  differently  colored  (Fig.  106),  some  appearing 
very  dark,  others  less  dark,  others,  still,  brilliant.  This  heterogeneousness  in  the 
appearance  of  sections  through  adjacent  grains  is  due  to  the  fact  elucidated  above 
that  each  grain,  i.e.  each  allotrimorphic  crystal,  is  made  up  of  crystalline  matter  similarly 
oriented  in  the  same  grain  but  differently  in  different  grains.  Bearing  this  in  mind  the 
dissimilarity  of  coloration  between  contiguous  grains  is  readily  explained,  for  if  the 


Fig.  107.  —  Typical  etching 
figures  of  pure  metals. 
(Gulliver.) 


Fig.  108.  —  Part  of  a  crystalline  grain  of  bismuth. 
Magnified  5  diameters.      (Gulliver.) 


crystalline  matter  of  any  individual  grain  is  so  oriented  that  it  reflects  the  incident 
light  into  the  microscope-tube,  that  grain  will  appear  bright,  while,  on  the  contrary, 
if  its  orientation  is  such  that  the  light  is  reflected  outside  the  microscope,  the  corre- 
sponding grain  will  appear  dark.  By  slightly  inclining  the  sample  in  various  direc- 
tions, or  by  rotating  it,  some  of  the  grains  that  were  bright  become  dark,  being,  so  to 
speak,  extinguished,  while  some  of  the  dark  grains  become  brightly  illuminated, 
because  in  so  doing  we  change  the  direction  of  the  light  reflected  by  each  individual 
grain  section.  Similar  results  are  obtained  by  changing  the  direction  of  the  incident 
light.  The  kaleidoscopic  effect  just  described  affords  a  conclusive  proof  of  the  crys- 
talline nature  of  metals  and  of  the  correctness  of  the  explanation  offered  to  account 
for  the  dissimilar  appearance,  as  to  color,  of  contiguous  grains. 

Cubic  Crystallization  of  Metals.  —  Etching  Pits.  —  By  still  deeper  etching  of  the 
polished  surfaces  of  pure  metals,  it  is  sometimes  possible  to  bring  out  clearly  the 
crystalline  character  of  the  individual  grains  (see  Fig.  107).  The  figures  thus  outlined, 
in  reality  small  cavities,  are  often  called  "etching  pits"  or  "etching  figures."  These 
figures  frequently  correspond  to  sections  of  cubes  or  of  geometrical  solids  derived 


CHAPTER   IV  — PURE   METALS  91 

from  the  cube,  indicating  that  most  metals  crystallize  in  the  cubic  system  (also  called 
regular,  or  isometric,  or  monometric  system).1 

In  Figure  108  is  seen  after  Gulliver  part  of  a  single  grain  of  bismuth.  It  is  clearly 
made  up  of  small  crystals  of  geometric  form. 

Summary.  —  Summing  up  the  indications  obtained  by  the  microscopical  exam- 
ination of  polished  and  etched  surfaces  of  pure  metals,  it  has  been  shown  (1)  that  a 
slight  etching  outlines  the  polygonal  boundaries  of  adjacent  crystalline  grains,  (2) 
,  that  a  deeper  etching  imparts  different  colorations  to  the  various  polygons  or  grain 
sections,  a  phenomenon  which  is  due  to  the  constancy  of  crystalline  orientation  in 
any  individual  grain  and  to  the  change  of  orientation  as  we  pass  from  one  grain  to 
the  next,  and  (3)  that  a  still  deeper  attack  often  brings  out  clearly  pits  of  distinct 
geometrical  forms,  often  cubic,  indicating  that  the  majority  of  metals  crystallize  in 
the  cubic  system.2 

The  Amorphous  Cement  Theory  and  the  Boundaries  of  Crystalline  Grains.  —  Ac- 
cording to  a  recent  theory  of  Rosenhain  and  Ewen  the  boundaries  between  adjacent 
grains  are  not  merely  surfaces  of  contact  but  narrow  spaces  filled  with  amorphous 
metal  or  cement  holding  the  grains  together.  This  hypothesis  is  an  extension  of 
Beilby's  theory  of  the  existence  of  an  amorphous  state  in  metals  caused  by  mechanical 
straining  as  outlined  elsewhere.  The  authors  argue  that  if  the  various  crystalline 
grains  were  merely  held  together  by  surface  contact  the  boundaries  should  be  weaker 
than  the  mass  whereas  on  the  contrary  fractures  generally  occur  through  the  crystal- 
line grains  instead  of  following  their  outlines.  Howe,  however,  pertinently  asks  why, 
following  the  author's  reasoning,  the  surfaces  of  contact  between  the  grains  and  the 
surrounding  amorphous  cement  membranes  should  not  be  as  many  planes  of  weakness 
which  the  fractures  should  follow.  In  other  words  why  should  the  contact  between  a 
crystalline  mass  and  an  amorphous  mass  of  the  same  metal  form  a  stronger  union 
than  contact  between  two  crystalline  masses  of  that  metal. 

Rosenhain  writes:  "where  the  constituent  crystals  of  a  metal  meet,  thin  films  of 
residual  liquid  will  remain  under  conditions  rendering  them  incapable  of  crystalliz- 
ing so  that  they  constitute  thin  films  of  undercooled3  liquid  or  amorphous  metal  act- 
ing as  intercrystalline  cement  .  .  .  When  two  differently  oriented  crystals  meet,  there 
must  be  a  region  between  the  two  orientations,  where  the  material  cannot  assume  the 
orientation  of  either  .  .  .  The  fundamental  fact  is  the  marked  strength  of  crystal 
boundaries  in  pure  metals." 

The  formation  of  amorphous  films  between  two  crystalline  grains  is  graphically 
represented,  according  to  Rosenhain,  in  Figure  109.  A  and  B  are  sections  through 
two  adjacent  grains  while  the  shaded  portion  between  them  represents  the  residual 
liquid  which  remains  amorphous  on  solidifying. 

The  existence  of  amorphous  crystalline  cement  accounts  for  the  greater  strength 

1  The  other  crystallographic  systems  arc  the  hexagonal,  the  tetragonal,  the  orthorhombic,  the 
monoclinic,  and  the  triclinic. 

2  We  have  other  indications  of  the  cubic  crystallization  of  metals  such  for  instance  as  the  hex- 
agonal character  of  many  of  the  polygons  which  crystallographers  consider  to  be  due  to  interfering 
cubes  and  octahedra  (the  octahedron  is  a  form  belonging  to  the  cubic  system).    Again,  under  favor- 
able crystallizing  conditions  nearly  perfect  cubes  have  been  obtained  in  the  case  of  several  metals. 

3  The  undercooling  of  a  substance  mcmis  its  cooling  to  a  temperature  lower  than  that  at  which 
a  certain  transformation  should  normally  take  place  without  inducing  that  transformation.     Water 
for  instance  cooled  below  its  freezing  point  without  causing  the  formation  of  ice  is  in  an  undercooled 
condition,  or  to  express  it  differently,  it  is  in  a  "metastable"  equilibrium. 


92 


CHAPTER  IV  — PURE   METALS 


and  toughness  of  a  fine-grained  structure  seeing  that  it  must  contain  a  greater 
quantity  of  strong  amorphous  cement.  It  will  be  shown  also  that  it  affords  an  ex- 
planation of  the  alteration  of  many  of  the  properties  of  pure  metals  when  subjected 
to  thermal  and  mechanical  treatments. 

The  difference  between  Beilby's  and  Rosenhain's  amorphous  cements,  however, 
should  not  be  overlooked:  the  former  results  from  the  severe  straining  of  crystallized 
metal,  the  latter  from  the  solidification  of  liquid  layers  enclosed  in  such  narrow  spaces 
that  they  cannot  assume  a  crystalline  orientation.  From  the  well-supported  con- 
tention, therefore,  that  Beilby's  cement  exists  and  that  it  is  strong,  hard,  and  brittle 
it  does  not  necessarily  follow  that  Rosenhain's  cement  possesses  similar  properties  or 
indeed  that  it  exists  at  all. 

Straining  of  Metals.  —  Slip  Bands.  —  Ewing  and,  later,  Ewing  and  Rosenhain 
through  some  skilfully  conducted  experiments  and  convincing  reasoning  have  re- 


A  B 

Fig.  109.  —  Section  through  side  crystalline  grains.     (Rosenhain.) 

vealed  the  character  of  the  strain  produced  in  a  pure  metal  by  the  action  of  a  stress 
which  may  eventually  cause  its  rupture. 

Pftished  sheets  of  metals  were  strained  very  gradually  while  being  examined 
under  the  microscope.  When  the  yield  point  is  reached,  i.e.  when  plastic  deforma- 
tion begins  to  occur,  black  lines  are  seen  to  cross  the  crystalline  grains  of  which 
the  metal  is  made  up.  These  lines  are  generally  quite  straight  and  are  parallel  in  the 
same  grains  but  have  different  directions  in  different  grains.  Figure  110  shows  the 
appearance  under  vertical  light  of  Swedish  iron  strained  by  tension,  magnified  400 
diameters. 

As  the  strain  increases  other  systems  of  lines,  inclined  to  the  first,  make  their 
appearance  and  eventually  two,  three,  and  even  four  systems  of  intersecting  lines 
may  be  seen  in  each  grain. 

These  lines  are  not  cracks  but  steps  in  the  surface,  which  steps  are  due  to  slips 
along  the  cleavage  or  gliding  planes  of  the  crystals.1  In  Figure  111,  AB  represents  the 

1  According  to  Humfrey  the  crystal  properties  of  cleavage  and  gliding  are  in  many  ways  distinct, 
and  do  not  necessarily  occur  with  maximum  readiness  along  the  same  crystallographic  planes.  This 
difference  probably  occurs  in  the  case  of  crystals  of  iron,  which  have  been  shown  to  possess  cubic 
cleavage  planes,  but  octahedral  gliding  planes. 


CHAPTER   IV  — PURE   METALS 


93 


The 


polished  surface  of  two  grains,  C  the  junction  line  between  these  two  grains. 
pull  takes  place  in  the  direction  of  the  arrows. 

Yielding  occurs  by  finite  amounts  of  slips  at  a  limited  number  of  places,  a,  b,  c, 
d,  e,  resulting  in  short  portions  of  inclined  cleavage  or  sliding  surface  appearing  black 


Fig.  110.  —  Slip  bands  in  Swedish  iron  strained 
by  tension.  Magnified  400  diameters.  (Ewing 
and  Rosenhain.) 

under  the  microscope,  because  they  do  not  send  back  any  light  into  the  tube.  By 
oblique  light  some  of  these  slip  bands  appear  black  while  others  are  bright.  When 
the  surface  is  rotated  some  of  the  bands  which  were  black  become  bright  and  vice 
versa  owing  to  the  change  of  their  position  in  regard  to  the  incidence  of  the  light, 


Before  straining. 


After  straining 

Fig.  111.  —  Diagram  illustrating  the  effect  of  strain 
upon  the  structure  of  metals  and  alloys.  (Ewing 
and  Rosenhain.) 

which  is  conclusive  proof  that  the  black  lines  are  not  cracks,  but  inclined  surfaces  as 
described  above. 

It  is  seen  then  that,  contrary  to  the  general  belief,  metals  remain  crystalline 
after  the  severest  strain,  and  that  the  flow  or  plastic  strain  in  metals  is  not  a  homo- 


94  CHAPTER   IV  — PURE   METALS 

geneous  shear  such  as  occurs  in  the  flow  of  viscous  fluids,  hut  is  the  result  of  a  limited 
number  of  separate  slips,  the  crystalline  elements  themselves  undergoing  no  deforma- 
tion. 

The  Amorphous  Cement  Theory  and  the  Straining  of  Metals.  —  Beilby  has  con- 
clusively shown  that  during  the  polishing  of  metals  with  very  fine  powders  thin 
layers  of  amorphous  metal  are  produced  considerably  harder  than  the  metal  itself 
and  necessarily  severely  strained.  Osmond  and  Cartaud  had  previously  made  a 
similar  observation.  If  it  be  accepted  that  an  amorphous  condition  may  be  produced 
on  the  surface  of  a  polished  specimen  through  the  mechanical  straining  of  the  polish- 
ing operation,  it  is  a  logical  conclusion  that  in  the  cold  working  of  metals  the  severe 
straining  which  that  operation  induces  will  likewise  produce  layers  of  amorphous 
metal  or  cement  between  the  surfaces  <>f  adjacent  crystals  having  undergone  the 
slipping  described  by  Ewing  and  Rosenhain.  To  the  formation  of  this  cement  may 


Fig.  112.  —  Twinnings  in  marble  (caused  by 
pressure).  Magnified  about  5  diameters. 
(Bayley.) 

be  ascribed  the  marked  changes  of  properties  accompanying  the  cold  working  of 
metals,  such  as  increased  tenacity  and  hardness,  decreased  ductility  and  density, 
etc. 

According  to  Tamman  and  others  the  crystalline  deformations  caused  by  me- 
chanical straining  are  sufficient  to  account  for  the  altered  properties  of  cold-worked 
metals,  it  being  unnecessary  to  conceive  the  formation  of  films  of  amorphous  cement 
covering  the  gliding  surfaces  after  the  slipping  process. 

Humfrey  having  observed  the  behavior  of  slip  bands  in  the  vicinity  of  the  grains 
boundaries  and  assuming  the  existence  of  intercrystalline  cement,  writes:  "All  the 
observations  demonstrate  the  resistance  to  plastic  deformation  by  slip  which  occurs 
at  the  crystal  junctions,  and  suggest  the  idea  that  each  crystal  is  surrounded  by  and 
firmly  attached  to  a  continuous  skin,  which,  while  hard  and  plastically  undeform- 
able,  is  yet  thin  and  capable  of  elastic  bending  and  stretching  .  .  .  The  state  of  affairs 
can  best  be  conceived  by  imagining  that  after  overstrain  each  crystal  is  enclosed  and 
held  within  a  stretched  skin  holding  it  so  as  to  oppose  further  strain  in  the  same 
direction." 


CHAPTER   IV —  PURE    METALS 


95 


Twinning  and  Twin  Crystals.  —  By  twinning  is  meant  the  grouping  of  two  or 
more  crystals  or  parts  of  crystals  in  such  a  way  that  they  are  symmetrical  to  each 
other  with  respect  to  a  plane  between  them  (the  twinning  plane)  which  plane,  how- 
ever, is  not  a  plane  of  symmetry.  Twins  result  generally  from  a  portion  of  a  crystal 


Fig.  113.  —  Twinnings  in  copper  produced  by  straining  followed  by  annealing. 
Magnified  12.")  diameters.     (F.  C.  Langenberg  in  the  author's  laboratory.) 


Fig.  114.  —  Lines  of  Neumann  in  a  low  carbon  steel  sheet.     Magnified  100  diameters. 


turning  by  a  definite  angle,  say  180  (leg.  They  are  sometimes  produced  by  straining 
alone  (Fig.  112),  when  they  are  called  mechanical  twins,  but  more  often  by  straining 
followed  by  annealing.  Many  metals  after  such  treatments  contain  numerous  twins 
(Fig.  113).  The  production  of  twin  crystals  in  the  complete  absence  of  strain  is 
doubtful. 


96  CHAPTER  IV  — PURE   METALS 

Lines  of  Neumann.  — •  The  crystalline  grains  of  metals  are  sometimes  found  to 
be  crossed  by  a  number  of  parallel  lines  or  bands  (Fig.  114)  called  "Lines  of  Neu- 
mann" and  which  might  at  first  be  taken  for  slip  bands  but  which,  according  to 
Osmond  and  Cartaud  and  others,  are  undoubtedly  mechanical  twins.  They  follow 
the  orientation  of  the  grains.  Lines  of  Neumann  can  be  distinguished  from  slip  bands 
by  repolishing  and  etching  the  sample,  when  slip  bands  do  not  reappear  while  the 
lines  of  Neumann  do. 

Influence  of  Mechanical  Treatment.  —  Metals  are  frequently  subjected  to  power- 
ful pressure  exerted  by  rolls,  presses,  hammers,  etc.,  with  a  view  of  producing  metallic 
objects  of  desired  shapes.  This  treatment  affects  the  structure  and,  therefore,  the 
properties  of  the  metal.  Roughly  speaking  such  vigorous  kneading  has  a  tendency 
to  reduce  the  size  of  the  final  grains,  either  through  preventing  the  formation  of 
large  grains  or  by  breaking  up  or  distorting  preexisting  grains.  A  smaller  grain  in 
turn  generally  implies  greater  ductility  (provided  it  be  not  distorted)  and  often  greater 
strength.  The  effect  of  work  upon  the  structure  and  properties  of  commercial  iron 
and  steel  will  be  duly  considered. 

Influence  of  Thermal  Treatment.  —  The  size  of  the  crystalline  grains  of  pure 
metals  varies  in  different  metals  even  when  cast  and  cooled  under  identical  condi- 
tions. Their  dimension  is  generally  affected  also  by  the  rate  of  cooling  during  solidi- 
fication and,  therefore,  by  the  size  of  the  casting,  since  a  large  casting  will  cool 
more  slowly  than  a  smaller  one. 

The  common  belief  is  that  the  prolonged  exposure  of  pure  metals  to  a  high  tem- 
perature (annealing)  tends  to  enlarge  the  grains,  the  enlargement  being  the  greater 
the  higher  the  temperature,  the  longer  the  time  of  exposure,  and  the  slower  the 
cooling.  While  such  growth  undoubtedly  takes  place  in  the  case  of  commercial  and, 
therefore,  impure  metals,  at  least  after  straining,  it  is  held  by  some  metallurgists 
that  in  absolutely  pure  metals  the  grain  will  not  grow  on  annealing  even  after 
straining. 

This  view  is  based  upon  a  theory  brilliantly  conceived  by  Ewing  and  Rosenhaiu  and  supported 
by  the  results  of  skilfully  conducted  experiments.  These  scientists  argue  that  even  so-called  pure 
metals  always  contain  a  certain  amount  of  impurities,  and  that  even  a  very  minute  amount  of  im- 
purity would  suffice  to  form  a  thin  but  practically  continuous  film  of  eutectic  in  the  crystalline 
boundaries.  They  contend  "that  there  is  constant  diffusion  from  the  surface  of  the  crystal  into 
the  eutectic  and  equally  constant  re-deposition  of  metal  upon  the  crystal  from  the  eutectic.  If  there 
are  several  crystals  in  contact  with  the  same  eutectic,  there  will  be,  under  some  conditions,  a  state 
of  dynamic  equilibrium  between*  them,  the  amount  dissolved  from  each  being  exactly  counter- 
balanced by  the  amount  deposited  upon  it;  if,  however,  there  is  any  difference  in  their  solution  pres- 
sure in  respect  to  the  eutectic,  then  the  less  soluble  will  grow  at  the  expense  of  the  more  soluble. 
The  metal  constituting  the  eutectic  films  being  much  nearer  its  melting-point  than  the  rest  of  the 
mass,  would  thus  be  favorable  to  comparatively  rapid  diffusion,  but  the  rate  of  such  diffusion,  and, 
consequently,  the  rate  of  growth  of  crystals,  would  be  enormously  increased  by  heating  the  metal 
to  a  temperature  above  the  melting-point  of  the  eutectic  in  question." 

The  theory  proposed  depends  upon  the  existence  of  a  difference  in  the  solubility  of  the  two  crys- 
tal faces  in  contact  with  the  eutectic  film.  The  only  difference  between  these  two  faces  is,  appar- 
ently, in  the  orientation  of  the  crystalline  elements,  but  this  difference  is  sufficient,  in  the  authors' 
opinion,  to  produce  a  difference  in  their  rate  of  solution  in  the  eutectic  film,  seeing  that  it  results  in 
such  marked  difference  in  their  solubility  in  the  etching  acid,  which,  as  is  well  known,  attacks  some 
grains  much  more  readily  and  deeply  than  neighboring  ones  whose  elements  have  another  orienta- 
tion. To  account  for  the  influence  of  the  orientation  of  the  elements  upon  the  solubility  of  the  crys- 
tals, the  authors  suggest  to  extend  to  alloys  the  electrolytic  theory  of  solution.  "Such  differential 
actions,"  they  say,  "may  most  probably  be  attributed  to  differences  of  electrical  potential  in  the 


CHAPTER   IV  — PURE   METALS  97 

surfaces  involved.  If  we  accept  this  view  of  the  mutter,  then  the  diffusion  across  films  of  eutectic' 
becomes  a  case  of  electrolysis." 

This  theory  explains  why  only  strained  crystals  of  the  metals  examined  will  grow,  while  un- 
strained crystals  show  no  tendency  to  change,  even  at  high  temperatures.  "The  explanation,  on  the 
electrolytic  theory,  is  that  in  the  unstrained  state  the  crystals  are  surrounded  by  practically  con- 
tinuous films  of  eutectic,  and  that  electrolysis  only  becomes  possible  when  severe  distortion  has 
broken  through  these  films  in  places,  allowing  the  actual  crystals  to  come  into  contact;  the  electro- 
lytic circuit  would  then  be  for  each  pair  of  crystals,  from  one  crystal  to  the  other  by  direct  contact 
and  back  through  the  eutectic  film." 

If  the  authors'  conception  be  true,  recrystallization  by  annealing  in  a  perfectly  pure  metal 
would  not  occur  but.  as  they  rightly  say,  it  is  almost  hopeless  to  obtain  a  sample  of  metal  suffi- 
ciently pure  to  prove  or  disprove  the  theory.  They  argue,  however,  that  if  the  growth  of  crystals 
is  due  to  the  presence  of  a  eutectic  film  between  them,  the  absence  of  such  film  would  be  a  barrier 
to  all  such  growth,  and  that  a  weld  between  two  clean-cut  surfaces  should  show  no  growth  of 
crystal  across  the  weld.  This  they  actually  proved  to  be  so  in  the  case  of  lead. 

If  they  are  right,  it  likewise  follows  that  in  metals  contaminated  by  impurities  with  which 
they  form  solid  solutions  there  should  be  no  growth  of  grains  on  annealing,  because  of  the  absence 
of  the  needed  eutectic  film.  , 

The  remarkable  crystalline  growth  of  very  low  carbon  steel  after  severe  straining 
followed  by  annealing,  at  suitable  temperatures,  described  in  another  chapter,  is  a 
.striking  instance  of  the  action  of  straining  (cold  working)  in  promoting  grain  growth 
in  subsequent  annealing. 

Notwithstanding  Ewing  and  Rosenhain's  strong  argument,  satisfactory  evi- 
dences are  still  lacking  in  support  of  the  contention  that  both  straining  and  the 
presence  of  impurities  are  necessary  conditions  to  grain  growth  on  annealing.  So 
far  as  the  matter  has  been  investigated  it  does  seem  that  the  grain  of  a  pure  metal 
will  not  grow  unless  it  has  been  previously  strained,  but  that  it  must  also  contain 
some  eutectic  forming  impurities  has  not  been  satisfactorily  demonstrated. 

Amorphous  Cement  Theory  vs.  the  Heat  Treatment  of  Pure  Metals.  —  It  is  be- 
lieved by  Rosenhain  and  Ewen  that  the  amorphous  films  cementing  together  the 
crystalline  grains  of  pure  metals  act  as  a  vehicle  for  crystal  growth  under  suitable  tem- 
perature conditions.  This  intercrystalline  amorphous  cement  might  play  the  part 
ascribed  to  eutectic  films  in  Ewing  and  Rosenhain's  earlier  theory.  While  according 
to  the  former  theory  the  grains  of  strictly  pure  metals  could  not  grow  on  annealing, 
even  after  straining,  owing  to  the  absence  of  eutectic  films,  the  amorphous  cement 
theory  permits  such  growth  and  this  is  in  better  harmony  with  observed  facts. 

The  annealing  of  cold-worked  metal  should  result  in  the  transformation  of  some 
of  the  strong  but  hard  and  brittle  cement  resulting  from  the  cold  working,  as  pre- 
viously explained,  into  crystallized  metal  and  this  should  be  accompanied  by  de- 
creased hardness  and  increased  ductility,  thus  accounting  for  the  well-known  influ- 
ence of  annealing  on  cold-worked  metal. 

Impurities.  —  It  is  well  known  that  the  addition  of  surprisingly  small  amounts  of 
impurities  or  foreign  substances  often  affect  very  greatly  some  of  the  most  important 
properties  of  metals,  such  as  their  strength,  ductility,  fusibility,  electrical  conduc- 
tivity, etc.,  and  we  naturally  look  for  correspondingly  marked  changes  of  structure. 
In  order  to  understand  this  important  influence  of  impurities  upon  the  properties  of 
metals  it  will  be  necessary  to  consider  at  some  length  the  nature  of  the  union  which 
exists  between  the  metal  and  the  impurity.  Let  us  first  note  that  by  impurity  we 
mean  a  very  small  proportion  of  some  foreign  substance  which  may  be  any  other 
metal,  a  metalloid,  or  a  definite  compound. 


98  CHAPTER  IV  — PURE  METALS 

The  metal  or  metalloid  contaminating  the  metal  may  (1)  remain  uncombined  or 
(2)  it  may  combine  with  some  (generally  a  small  amount)  of  the  metal  to  form  a 
definite  chemical  compound.  The  uncombined  contaminating  metal  or  metalloid 
or  resulting  chemical  compound  may  then  (a)  be  soluble  in  the  solid  metal  forming 
with  it  a  "solid  solution"  or  (6)  be  insoluble  in  the  solid  metal  in  which  case  it  is 
rejected  by  the  crystalline  grains,  in  the  form  of  an  eutectic  alloy. 

The  meaning  of  the  expressions  "solid  solutions"  and  "eutectic  alloys"  should 
now  be  explained.  As  Professor  Howe  has  well  expressed  it  the  essential  features  of 
an  ordinary  liquid  solution  are  (1)  a  complete  merging  of  the  constituents  and  (2)  in 
indefinite  proportions.  By  complete  merging  is  meant  a  union  so  intimate  that  the 
separate  existence  of  the  constituents  cannot  be  detected  by  any  physical  means, 
such  for  instance  as  microscopic  examination  under  the  highest  magnification.  The 
homogeneity  of  the  substance  is  such  that  it  resists  any  physical  attempt  at  breaking 
it.  The  merging  moreover  must  remain  complete  and  absolute  for  varying  propor- 
tions of  the  constituents,  for  it  is  evident  that  if  it  existed  only  for  certain  well-defined 
proportions  of  the  component  parts,  the  resulting  substance  would  be  of  the  nature 
of  a  definite  chemical  compound  and  not  of  a  solution. 

Bearing  in  mind  these  characteristics  of  ordinary  solutions,  we  find  that  in  some 
substances,  while  passing  from  the  liquid  to  the  solid  state,  the  constituents  remain 
completely  merged  and  in  indefinite  proportions.  The  essential  characteristics  of 
liquid  solutions  are  retained  in  the  solid  state.  Hence  the  name  of  solid  solutions  given 
to  such  substances.  A  common  and  excellent  example  of  solid  solutions  is  found  in 
the  case  of  glass  in  which  the  three  usual  constituents,  silica,  lime,  and  alkali,  are  so 
completely  merged  that  their  existence  cannot  be  detected  by  physical  means;  the 
microscopical  examination  of  glass  under  the  highest  magnification  fails  to  reveal  the 
presence  of  its  component  parts.  Glass  on  solidifying  passes  from  the  condition  of  a 
liquid  solution  to  that  of  a  solid  solution.  Many  metals  likewise  form  on  solidifying 
solid  solutions,  i.e.  they  solidify  into  a  mass  so  absolutely  homogeneous  that  the 
identity  of  the  component  metals  is  entirely  lost.  The  union  between  some  metals 
and  metalloids  also  frequently  forms  solid  solutions. 

It  is  held  by  some  crystallographers  that  in  order  to  form  solid  solutions  the  unit- 
ing subtances  must  be  "isomorphous,"  that  is,  must  be  capable  of  yielding  crystals 
of  the  same  form,  hence  the  name  of  "isomorphous  mixtures"  frequently  given  to  solid 
solutions.1  The  homogeneous  crystals  formed  by  solid  solutions  are  often  called 
"mixed  crystals"  —and  that  expression  frequently  used  as  an  equivalent  for  solid 
solution.  There  are  some  crystallographers,  however,  who  believe  that  isomorphism 
of  the  constituents  is  not  essential  to  the  formation  between  them  of  solid  solutions, 
and  that  the  use,  therefore,  of  isomorphous  mixtures  as  synonymous  of  solid  solu- 
tion is  not  warranted.  The  use  of  the  expression  mixed  crystals  is  likewise  to  be 
discouraged  because  it  suggests  a  mixture,  and,  therefore,  heterogeneity,  which  is 
precisely  contrary  to  the  nature  of  solid  solutions. 

Considering  now  those  impurities,  whether  metals,  metalloids,  or  definite  com- 
pounds, which  form  solid  solutions  with  the  metal  they  contaminate,  it  is  found 

1  If  isomorphism  favors  the  formation  of  solid  solutions,  as  it  undoubtedly  does,  seeing  that 
most  metals  are  isomorphous,  we  naturally  infer  that  they  will  readily  form  solid  solutions.  We 
now  know  that  such  is  the  case,  for  if  metals  are  not  generally  soluble  in  each  other  (when  solid)  in 
all  proportions  there  are  few  instances  of  metals  entirely  insoluble  in  each  other  in  the  solid  state. 
The  formation  of  solid  solutions  between  metals  is  therefore  very  frequent. 


CHAPTER  IV  — PURE  METALS  99 

as  might  have  been  expected  that  their  presence  has  no  great  influence  upon  the 
character  of  the  structure.  Suitably  prepared  surfaces  of  such  impure  metals  still 
exhibit  the  polygonal  network  structure  characteristic  of  pure  metals.  The  small 
polyhedra  of  which  the  impure  metal  is  composed,  however,  are  now  allotrimorphic 
crystals  of  a  solid  solution  instead  of  a  pure  metal.  While  the  character  of  the  struc- 
ture remains  the  same,  the  dimension  of  the  grains  may  be  markedly  affected,  by  the 
presence  of  a  small  amount  of  impurity  forming  a  solid  solution  with  the  metal. 

The  second  group  of  impurities,  namely  those  foreign  substances,  whether  they 
remain  or  not  uncombinecl,  which  do  not  form  solid  solutions  with  the  contaminated 
metal  may  usually  be  readily  detected  under  the  microscope  as  they  are  generally 
rejected  to  the  grain  boundaries  during  the  process  of  solidification  (or  afterwards) 
as  shown  in  Figure  115.  These  insoluble  impurities  are  not,  however,  rejected  as  such 
by  the  crystalline  grains,  but  on  the  contrary  unite  mechanically  with  a  small  amount 


Fig.  115.  —  Gold  containing  0.20  per 
cent  lead.  Magnified  100  diameters. 
(Andrews.) 


of  the  metal  to  form  what  is  known  as  an  "eutectic  alloy,"  that  is  an  alloy  of  lowest 
melting-point,  and  it  is  this  alloy  which  is  expelled  by  the  solidifying  grains.  The 
formation  and  nature  of  eutectic  alloys  will  be  considered  at  greater  length  in  a  sub- 
sequent chapter. 

It  will  be  apparent  that  those  contaminating  substances  which  fail  to  be  dissolved 
by  the  metal,  may  form  actual  membranes  surrounding  each  grain,  the  membranes 
being  of  the  nature  of  an  eutectic  alloy.  As  might  be  anticipated,  the  presence  of  such 
membranes,  whether  continuous  or  not,  have  generally  a  very  marked  influence  upon 
the  properties  of  the  metals,  frequently  decreasing  their  ductility,  weldability,  electrical 
conductivity,  etc.,  and  often  increasing  their  fusibility,  hardness,  etc. 

The  rejection  during  solidification  and  subsequent  cooling  of  those  impurities 
which  fail  to  be  retained  in  solid  solution  by  the  metal,  to  the  grain  boundaries  or 
other  crystallographic  planes,  reveals  the  crystalline  forms  of  the  grains  themselves. 
The  location  of  these  impurities  affords  additional  evidence  of  the  cubic  crystalliza- 
tion of  metals.  It  will  be  shown  later  that  the  cubic  crystallization  of  iron  is  in  this 
way  clearly  revealed. 


100  CHAPTER   IV  —  I'L'KK   MKTALS 

The  above  remarks  are  of  a  very  general  character  and  refer  more  especially  to 
the  behavior  of  impurities  while  the  metal  solidifies.  In  the  majority  of  cases  no 
further  changes  take  place  in  the  nature  of  the  constituents  as  the  metal  cools  to 
atmospheric  temperature,  i.e.  the  constituents  formed  on  solidification  are  those 
found  in  the  metal  after  complete  and  slow  cooling.  In  some  instances,  however, 
and  notably  in  the  case  of  iron  and  its  usual  impurities,  carbon,  silicon,  phosphorus, 
manganese,  and  sulphur,  some  important  changes  take  place  at  temperatures  con- 
siderably below  the  solidification  point  of  the  metal  which  will  be  duly  described  at 
the  proper  time. 

Some  writers  believe  that  in  slightly  impure  metals  the  impurities  may  form 
nuclei  from  which  crystal  growth  may  s.tart,  thus  accounting  for  the  apparent  fact  that 
slightly  impure  metals  have  often  smaller  grains  than  metals  of  greater  purity.  This 
assumption  demands  an  early  solidification  of  small  particles  of  impurities  within  the 
liquid  metal  and,  therefore,  can  be  true  only  in  the  case  of  foreign  substances  having 
a  higher  melting  point  than  the  metal  itself.  It  is  not  easily  conceived  how  it  can  be 
applied  to  impurities  forming  solid  solutions  with  the  metal,  since  they  then  solidify 
together,  nor  to  impurities  forming  eutectic  alloys,  since  such  alloys  constitute  the 
constituent  of  last  solidification. 


CHAPTER  V 

PURE   IRON 

Chemically  pure  iron  is  not  a  commercial  product.  It  can  only  be  obtained  in 
small  quantities  by  carefully  conducted  laboratory  manipulations  when,  even  with 
the  most  refined  care,  it  is  quite  impossible  to  produce  it  absolutely  pure.  Until 
quite  recently  the  purest  commercial  iron  was  of  Swedish  origin  and  contained  as 
much  as  99.8  per  cent  of  iron.  A  commercial  product  known  as  "American  Ingot 
Iron"  l  is  now  manufactured  which  the  makers  claim  to  contain  99.94  per  cent  of 


Fig.  11(>.  —  Electrolytic  iron.      Magnified  75  diameters.     Slightly  etched, 
i  Sin-rani  Cowper-Coles.) 


iron.  Iron,  or  rather  very  low  carbon  steel,  of  a  high  degree  of  purity  is  also  produced 
at  the  present  time  through  refining  in  electric  furnaces.  Finally  iron  has  been  ob- 
tained in  relatively  large  quantity,  by  electrolytic  deposition,  of  a  degree  of  purity 
exceeding  99.9  per  cent  iron. 

1  The  expression  "ingot"  iron  is  applied  to  iron  containing  very  little  carbon,  obtained  molten 
and,  therefore,  cast,  after  removal  from  the  furnace,  whereas  by  wrought  iron  is  meant  iron  (also 
generally  low  in  carbon)  obtained  pasty  and,  consequently,  always  containing  a  certain  amount  of 
slag.  Ban-ing  the  presence  of  slag  in  wrought  iron,  both  ingot  iron  and  wrought  iron  may  have 
identical  chemical  compositions.  The  expression  ingot  iron  is  seldom  used  in  the  United  Slates, 
where  iron  obtained  in  a  liquid  state,  and  containing  little  carbon,  is  called  low  or  very  low  carbon 
steel,  or  mild,  very  mild,  extra  mild  steel,  or  again  soft,  very  soft,  dead  soft  steel. 

101 


102 


CHAPTER  V  — PURE  IRON 


Microstructure.  —  When  a  sample  of  nearly  pure  iron  is  suitably  prepared  and 
examined  under  the  microscope  some  regions  can  readily  be  found  absolutely  free 
from  carbon  and  slag  and  exhibiting,  therefore,  the  structure  of  the  pure  metal. 
Such  structure  is  illustrated  in  Figure  116  after  a  slight  etching  of  the  polished  surface. 
It  will  be  noted  that  it  is  similar  to  the  structure  of  pure  gold  and  of  pure  copper 


Fig.  117.  —  Ferrite  grains.     Natural  size.    Etched  10  minutes 
in  nitric  acid  (1  to  10  water).    (Stead.) 

described  in  the  preceding  chapter:  like  gold  and  copper  and,  indeed,  like  most  metals, 
iron  is  made  up  of  polyhedral  crystalline  grains  (allotrimorphic  crystals).  Upon 
deeper  etching  the  dissimilarity,  as  to  coloration,  of  adjacent  iron  grains  is  clearly 
brought  out  as  shown  in  Figure  117.  As  explained  in  Chapter  IV,  this  appearance 
is  due  to  the  fact  fhat  the  grains  of  iron  are  composed  of  crystalline  elements  having 
the  same  orientation  in  the  same  grain  but  different  ones  in  different  grains. 


CHAPTER  V  — PURE   IRON 


103 


As  explained  in  the  preceding  chapter  it  is  believed  by  some  that  the  crystalline 
grains  of  iron  like  those  of  other  pure  metals  are  surrounded  by  thin  membranes  of 
amorphous  cement  (the  network  in  Fig.  116)  holding  the  grains  together,  this  amor- 
phous metal  being  harder  and  stronger  but  less  ductile  than  the  crystalline  grains 
themselves.  Before  this  theory  had  been  advanced  it  was  universally  assumed  that 


Fig.  US.  — Etching  pits  in  iron. 
(Desch.) 


Fig.  120.  —  Silicon  steel,  4.5  per 
cent  silicon.  Magnified  60 
diameters.  Part  of  a  single 
grain.  Etched  3  hours  in 
nitric  acid  (1  to  10  water). 
(Stead.) 


Fig.  119.  —  Etching  pits  in  ferrite.     (J.  F. 
Hoyland,  Correspondence  Course  student.) 


Fig.  121.  —  Cubic  crystals  of  iron.  Magnified 
250  diameters.  Obtained  through  the  reduc- 
tion of  forrous  chloride.  (Osmond.) 


the  grains  were  held  together  by  surface  contact  alone  or  cohesion  and  that  the  net- 
work revealed  on  etching  merely  indicated  sections  through  these  contact  surfaces 
made  apparent  owing  to  slight  differences  of  solubility  of  adjacent  grains  (because  of 
their  different  orientation)  and  resulting  in  slight  differences  of  level  after  etching. 

Cubic  Crystallization  of  Iron.  —  A  still  deeper  etching  indicates  clearly  the  cubic 
character  of  the  crystallization  of  pure  iron.  This  is  illustrated  diagrammatically  in 
Figure  118  and  by  means  of  a  photomicrograph  in  Figure  119.  It  will  be  noted  that 


104  CHAPTER   V  — PUKE    IRON 

the  etching  pits  are  similarly  oriented  in  the  same  grain  hut  that  the  orientation  in 
adjacent  grains  differs.  As  seen  in  Figure  118,  the  etching  figures  may  appear  as 
triangular  wedges.  This  occurs  when  the  section  cuts  the  small  cubes  of  a  grain  at  a 
certain  angle,  i.e.  when  it  cuts  obliquely  a  corner  of  each  cube.  This  cubic  structure  is 
further  illustrated  in  a  remarkable  manner  in  Figure  120,  in  the  case  of  iron  contain- 
ing 4i/£  per  cent  of  silicon  after  etching  three  hours  in  dilute  nitric  acid.  The  photo- 
graph shows  a  portion  of  a  single  grain,  hence  the  constancy  of  orientation  noted. 
The  presence  of  so  much  silicon  apparently  favors  the  development  of  a  coarse  cubic 
crystallization. 

Osmond,  through  the  reduction  of  ferrous  chloride  and  the  crystallization  of  the 
resulting  metallic  iron,  obtained  perfect  isolated  cubic  crystals  (Fig.  121).  Finally 
almost  perfect  cubes  have  been  separated  by  Stead  from  a  large  granule  of  phos- 


Fig.  122.  —  Cubic  crystals  of  phosphoretic  iron. 
Magnified  5  diameters.  Phosphorous  0.7i3  per 
cenl,  carbon  0  per  cent.  (Stead.) 

phoretic  iron  (Fig.  122).  Another  indication  of  the  cubic  crystallization  of  iron  is 
found  in  the  occurrence  of  large  crystallites  (Fig.  123),  generally  resembling  pine  trees, 
in  cavities  of  large  castings  of  iron  and  steel,  under  conditions,  therefore,  favorable  to 
the  free  development  of  crystals,  these  crystallites  being  composed  of  small  octa- 
hedra,  a  crystalline  form  of  the  cubic  system.  Finally  it  will  be  shown  later  that 
the  structural  location  of  some  of  the  impurities  generally  present  in  commercial 
iron  affords  further  proof  of  the  cubic  crystallization  of  iron. 

Ferrite.  —  Mineralogical  names  have  been  given  to  the  constituents  of  iron  and 
steel,  and  pure  iron,  or  rather  carbonless  iron,  considered  as  a  microscopical  con- 
stituent has  been  called  "ferrite,"  a  name  suggested  by  Prof.  H.  M.  Howe  and 
universally  adopted.1  Pure  iron,  therefore,  is  composed  of  polyhedral  crystalline 

1  This  constituent  was  called  "free  iron'1  by  Sorby,  who  was  Hie  first  scientist  to  describe  the 
microscopical  structure  of  iron  and  steel. 

For  further  description  of  the  nature  and  properties  of  ferrite  see  the  report  of  the  Committee 
of  the  International  Association  for  the  Testing  of  Materials,  "On  the  Nomenclature  of  the  Micro- 
scopic Substances  and  Structures  of  Steel  and  Cast  Iron"  in  the  Appendix.  Stead  has  suggested  that 
when  ferrite  consists  only  of  pure  iron  it  should  be  called  "ferro-ferrito." 


CHAPTER    V  — PURE   IRON 


105 


Fig.  123.  —  Iron  crystallite  about  half 
natural  size.     (Tschernoff.) 


106  CHAPTER   V  — PURE   IRON 

grains  of  ferrite.  It  will  be  seen  in  subsequent  chapters  that  the  ferrite  of  commercial 
grades  of  iron  and  steel  is  not  pure  iron,  but  rather  iron  holding  in  solid  solution 
small  amounts  of  silicon,  phosphorus,  and  possibly  other  impurities. 

Allotropy  of  Iron.  — •  The  study  of  the  crystallization  of  iron  is  complicated  by 
the  existence  of  several  allotropic  forms  of  that  metal. 

Physicists  and  chemists  do  not  agree  as  to  the  exact  meaning  of  allotropy,  still 
less  as  to  the  phraseology  by  which  it  should  be  denned.  Allotropy  certainly  implies 
marked  and  sudden  reversible  changes  taking  place  in  some  of  the  properties  of  a 
substance  at  certain  critical  temperatures  excluding  formation  or  dissociation  of 
chemical  compounds  and  changes  of  state  although  the  latter  might  be  considered  as 
major  instances  of  allotropy.  Opinions  differ,  however,  as  to  whether  a  well-defined 
change  in  a  single  property  constitutes  a  proof  of  allotropy,  while  some  scientists 
insist  that  unless  a  crystallographic  change  is  observed  at  the  critical  temperature 
the  transformation  cannot  be  considered  as  an  allotropic  one.1  Again  it  is  claimed  by 
some  and  denied  by  others  that  a  spontaneous  evolution  of  heat  on  cooling  at  a  cer- 
tain critical  temperature  and  absorption  of  heat  on  heating  at  the  same  or  nearly 
the  same  temperature  is  sufficient  proof  of  an  allotropic  change.  Others  still  con- 
sider the  occurrence  of  a  sudden  dilatation  on  cooling  and  contraction  on  heating  as 
the  best  criterion  of  the  existence  of  allotropy.  Finally,  in  discussing  the  allotropy 
of  iron  it  has  been  attempted  by  some  writers,  arbitrarily  it  would  seem,  to  exclude 
magnetic  changes  from  those  indicative  of  allotropy. 

Many  substances  undergo  allotropic  changes.  It  is  a  matter  of  common  knowl- 
edge, for  instance,  that  sulphur  exists  under  two  distinct  conditions,  namely,  as  pris- 
matic sulphur  and  as  octahedral  sulphur,  the  prismatic  form  being  the  one  stable 
above  95.6  deg.  C.  and  the  octahedral,  the  stable  form  below  that  critical  tempera- 
ture. On  heating  octahedral  sulphur  it  begins  to  change  into  the  prismatic  form  at 
the  temperature  of  95.6  deg.  and,  likewise,  on  cooling,  prismatic  sulphur  begins  to 
pass  to  the  octahedral  form  at  that  temperature.  Many  of  the  physical  properties 
of  sulphur  (crystalline  form,  specific  heat,  heat  of  combustion,  etc.)  undergo  sudden 
changes  as  the  substance  passes  from  one  allotropic  form  to  another. 

At  those  critical  temperatures  which  mark  the  passage  of  one  allotropic  form  into 
another,  spontaneous  evolutions  of  heat  take  place  on  cooling  and  spontaneous  ab- 
sorptions of  heat  on  heating.  These  thermal  disturbances  indicate  a  change  of  in- 
ternal energy  which  when  not  accompanied  by  changes  of  state  or  by  chemical  changes 
are  in  the  author's  opinion  evidences  of  allotropy.  The  usual  method  of  detecting 
the  existence  of  such  thermal  critical  points  will  be  described  in  another  chapter. 

Osmond's  momentous  discovery  of  the  existence  of  two  thermal  critical  points  in 
pure  iron  respectively  at  about  768  and  900  deg.  C.  was  at  first  accepted  as  proving 
the  occurrence  of  iron  under  three  allotropic  varieties,  7  (gamma)  iron  stable  above 
the  upper  critical  point,  /3  (beta)  iron  stable  between  the  two  points,  and  a  (alpha) 
iron  stable  below  the  lower  point.  The  allotropic  character  of  gamma  and  alpha 
iron  is  still  universally  accepted,  and  indeed  has  been  firmly  established,  but  it  is 
contended  by  some  that  the  condition  of  iron  between  its  two  critical  points  does  not 
differ  allo tropically  from  its  condition  below  its  lower  critical  point;  in  other  words 
that  the  belief  in  beta  iron  as  a  distinct  allotropic  variety  should  be  abandoned.  This 
important  question  will  be  considered  at  greater  length  in  the  chapters  of  this  book 

1  The  property  of  some  substances  of  crystallizing  in  more  than  one  form  is  called  polymorphism. 
The  expression,  however,  should  not  be  used  .as  an  equivalent  of  allotropy. 


CHAPTER  V  — PURE   IRON  107 

dealing  with  the  Thermal  Critical  Points  of  Iron  and  Steel,  with  the  Hardening  of 
Steel  and  with  the  Equilibrium  Diagram  of  Iron-Carbon  Alloys,  when  it  will  be 
shown  that  these  opposite  views  result  apparently  from  different  conceptions  as  to 
the  nature  of  allotropy.  For  our  present  purpose  it  will  be  assumed  that  iron  exists, 
as  first  described  by  Osmond,  under  three  allo tropic  conditions. 

Solidification  and  Crystallization  of  Pure  Iron.  —  As  should  be  expected  the  pas- 
sage of  one  allotropic  form  into  another  implies  corresponding  and,  generally,  sudden 
changes  in  many  of  the  physical  properties  of  iron.  Gamma,_be±a,  and  alpha  iron 
differ  widely  in  regard  to  many  of  their  physical  characteristics.  It  is  only  desired 
in  this  chapter,  however,  to  inquire  into  the  possible  differences  of  crystallization 


Fig.  124.  —  Twinnings  in  gamma  iron.     Mag- 
nified 200  diameters.     (Osmond.) 

which  may  exist  between  the  three  allotropic  conditions  of  iron,  leaving  for  later 
consideration  the  modification  of  the  other  properties. 

Osmond  and,  later,  Osmond  and  Cartaud  have  carefully  investigated  the  difficult 
problem  of  the  crystallization  of  the  different  allotropic  forms  of  iron.  Their  con- 
clusions were  (1)  that  the  three  allotropic  forms  of  iron  crystallize  in  the  cubic  sys- 
tem,1 (2)  that  octahedra  are  the  prevailing  crystalline  forms  of  gamma  iron,  (3)  that 
the  cube  is  the  prevailing  form  of  beta  and  of  alpha  iron,  (4)  that  beta  and  alpha  iron 
are  capable  of  forming  isomorphous  mixtures  (solid  solutions),  (5)  that  gamma  iron 
does  not  form  isomorphous  mixtures  with  beta  or  alpha  iron. 

We  also  have  the  statement  of  Osmond  that  the  transformation  of  gamma  iron 
into  beta  iron  appears  to  include  a  change  in  the  planes  of  symmetry,  at  least  in  car- 
burized  iron. 

Again  it  has  been  shown  by  Osmond  and  confirmed  by  other  investigators  that 
the  occurrence  of  twinnings  is  frequent  in  gamma  iron  (Fig.  124),  while  beta  and  alpha 
iron  are  free  from  it. 

1  In  Le  Chatelier's  opinion  there  is  no  proof  of  the  cubic  form  of  gamma  iron.  He  thinks  that 
the  facts  observed  are  contrary  to  that  hypothesis  and  that  it  is  more  probable  that  gamma  iron  is 
rhombohedral  or  orthorhombic. 


108  CHAPTER    V  — PURE    IRON 

It  follows  from  Osmond's  experiments  that  the  allotropy  of  iron  could  not  he 
proved  by  its  crystallography,  since  the  thermal  critical  points  are  not  accompanied 
by  changes  in  the  crystalline  form  of  iron.  While,  however,  in  the  instances  of  allo- 
tropy which  have  been  noted  and  studied  allotropic  changes  are  generally  accom- 
panied by  changes  of  crystalline  forms,  it  does  not  by  any  means  follow  that  any  allo- 
tropic transformation  must  necessarily  imply  a  crystalline  change. 

Bearing  in  mind  the  existence  of  three  allotropic  conditions  of  iron,  let  us  follow 
in  our  imagination  the  crystallization  of  iron  during  solidification  and  its  subsequent 
cooling  to  atmospheric  temperature.  On  solidifying  iron  crystallites  are  formed  con- 
sisting of  octahedral  crystals  of  gamma  iron.  Upon  further  cooling  below  the  solidi- 
fication point,  no  change  of  crystalline  form  should  take  place  unless  it  be  the  granula- 
tion described  by  Belaiew  (Chapter  XIII)  until  the  first  critical  temperature  (900 
deg.  C.)  is  reached  when  the  iron  changes  from  the  gamma  to  the  beta  condition. 

Does  this  allotropic  change  affect  the  preexisting  crystallization  of  gamma  iron 
or  does  it  consist  merely  in  a  transformation  in  situ  of  each  crystalline  grain  of  gamma 
iron  into  a  grain  of  beta  iron,  retaining  the  original  external  form  of  the  gamma 
grain,  and  leaving  undisturbed,  therefore,  the  polygonal  structure  observed  under  t he- 
microscope?  It  may  reasonably  be  supposed  that  the  allotropic  transformation  takes 
place  without  affecting  the  external  form  of  the  crystalline  grains,  but  in  view  of 
Osmond's  statement  that  the  octahedron  is  the  prevailing  crystalline  form  of  gamma 
iron  and  bearing  in  mind  that  the  small  crystals  revealed  by  suitable  etching  of  pure 
iron  are  generally  cubic,  we  naturally  infer  that  the  octahedral  character  of  each 
grain  of  gamma  iron  has  been  obliterated,  the  small  octahedral  elements  of  gamma 
iron  having  been  replaced  by  small  cubic  elements  of  beta  (and  later  of  alpha)  iron. 
These  conclusions,  however,  should  lie  accepted  with  reserve,  as  we  lack  evidences  of 
a  very  conclusive  character.  It  has  been  shown  that  the  change  of  gamma  iron  into 
beta  iron  is  accompanied  by  an  abrupt  expansion  of  the  cooling  metal  which  expan- 
sion is  followed  by  the  normal  contraction  of  cooling  substances.  We  infer  from 
this  that,  momentarily  at  least,  each  grain  of  beta  iron  occupies  more  space  than  its 
gamma  iron  progenitor. 

At  750  deg.  C.  or  thereabout,  the  iron  passes  from  the  beta  to  the  third  or  alpha 
form.  We  may  here  ask  the  same  questions  as  to  the  probable  effect  of  this  change 
upon  (1)  the  outward  form  of  each  beta  grain  and  (2)  upon  the  internal  crystalline 
structure  of  each  grain.  Since  both  beta  and  alpha  iron  crystallize  in  the  cubic  sys- 
tem, the  cube  being  the  prevailing  crystalline  form  of  both,  and  since,  according  to 
Osmond,  they  are  isomorphous,  that  is  capable  of  forming  solid  solutions,  it  seems 
probable  that  the  change  of  beta  to  alpha  iron  affects  neither  the  external  form  inn 
the  internal  crystalline  arrangement  of  the  beta  grains;  in  other  words  that  each 
small  cubic  element  of  beta  iron  is  converted  bodily  (although  probably  gradually 
and  not  abruptly)  into  a  cubic  element  of  alpha  iron. 

If  the  above  represents  the  real  mechanism  of  the  allotropic  changes,  it  will  lie 
evident  that  the  polyhedral  grains  revealed  through  the  etching  of  polished  sections 
of  pure  iron  were  formed  during  solidification  and  consisted  originally  of  gamma 
iron,  these  grains  having  retained  their  external  shape  while  undergoing  allotropic 
transformation,  but  having  probably  undergone  at  the  upper  critical  point  an  internal 
change,  the  octahedral  form  of  the  crystalline  elements  having  been  replaced  by  the 
cubic  form. 

Hosenhain  and  Humfrey  by  straining  a  bar  of  iron  heated  to  a  high  tempcr;ilure 


CHAPTER   V  —  PURE    IRON 


109 


at  the  center  (the  temperature,  therefore,  decreasing  towards  both  ends)  have  made 
evident  the  existence  of  three  distinct  kinds  of  distortions  with  sharp  lines  of  de- 
marcation between  them,  corresponding  in  all  probability  to  the  distortion  respect- 


12o.  —  Iron  before  straining.     (Rosenhain.) 


l\ 


Fig.  126.  —  Same  as  Fig.  125  but  strained.     (Rosenhain.) 

ively  of  gamma,  beta,  and  alpha  iron.  The  estimation  of  the  temperature  of  different 
portions  of  the  bar  by  means  of  fusible  salts  appears  to  sustain  the  authors'  conten- 
tion thus  furnishing  an  additional  support,  and  a  substantial  one,  to  Osmond's  bril- 
liant theory  of  the  allotropy  of  iron. 


110  CHAPTER  V  — PURE   IRON 

Nevertheless  information  of  a  more  positive  and  concordant  nature  is  still  needed 
to  settle  to  the  satisfaction  of  all  the  question  of  the  relation  between  the  thermal 
critical  points  of  pure  iron  and  possible  crystallographic  changes. 

Twinnings  and  Neumann  Lines.  —  It  has  been  already  stated  that  according  to 
Osmond  and  Cartaud  twin  crystals  frequently  occur  in  gamma  iron.  It  will  be 
shown  later,  however,  that  gamma  iron  which  exists  normally  only  above  the  upper 
critical  point  of  iron  requires  for  its  preservation  at  atmospheric  temperature  the 
presence  of  a  considerable  amount  of  carbon,  if  not,  also,  of  other  elements  such  as 
manganese  and  nickel.  The  twin  crystals  of  gamma  iron  shown  in  Figure  124  do  not 
refer  to  pure  gamma  iron  but  to  gamma  iron  containing  a  considerable  amount  of 
nickel. 

Iron  or  at  least  very  low  carbon  steel  not  unfrequently  exhibit  the  presence  of 
Neumann  lines  which  as  previously  stated  are  probably  mechanical  twins.  In  Fig- 
ure 114  an  interesting  instance  of  the  occurrence  of  such  lines  has  been  shown. 
.  It  has  been  mentioned  in  the  preceding  chapter  that  in  order  to  distinguish  be- 
tween slip  bands  and  Neumann  lines  the  specimen  should  be  repolished  and  again 
etched,  a  treatment  which  should  result  in  the  reappearance  of  the  Neumann  lines 
but  not  of  slip  bands  since  the  latter  are  merely  slight  differences  of  level  erased  by 
the  polishing  operation. 

Strains  and  Slip  Bands.  —  It  has  been  shown  by  Ewing  and  Rosenhain  that  like 
other  metals,  iron  when  subjected  to  a  strain  exceeding  its  elastic  limit  yields  through 
a  succession  of  small  crystalline  slips  readily  distinguishable  on  a  polished  surface 
as  one  or  more  systems  of  parallel  lines  (Figs.  125  and  126).  It  has  been  observed 
by  these  authors  that  this  slipping  occurs  most  readily  along  octahedral  planes  from 
which  it  would  seem  to  follow,  as  already  noted,  that  these  gliding  planes  are  not 
identical  to  cleavage  planes  since  the  latter  are  cubic. 

According  to  Humfrey  the  slip  bands  which  form  first  are  confined  almost  entirely 
to  the  central  parts  of  the  crystals  and  only  spread  gradually  towards  the  boundaries 
as  the  straining  becomes  more  severe  when  they  show  a  tendency  to  become  narrower 
and  to  bend  so  as  to  meet  them  at  a  smaller  angle. 

As  explained  in  Chapter  IV  it  is  believed  by  some  that  the  severe  straining  of  the 
iron  crystals  at  the  gliding  planes  must  result  in  the  formation  of  thin  layers  of  amor- 
phous metal  and  that  this  theory  affords  a  satisfactory  explanation  of  the  marked  in- 
fluence of  cold  working  on  the  physical  properties  of  iron. 

Influence  of  Mechanical  Treatment.  —  Like  that  of  other  metals  the  structure  of 
iron  is  affected  by  mechanical  work.  Undisturbed  cooling  being  a  necessary  condi- 
tion to  the  free  development  of  crystals,  it  will  be  evident  that  if  the  metal  be  vigor- 
ously worked,  that  is  subjected  to  powerful  mechanical  pressure,  while  cooling  from 
a  high  temperature,  the  formation  of  crystalline  grains  will  be  greatly  hindered  or 
preexisting  crystals  broken  or  distorted.  The  important  influence  of  work  upon  the 
structure  (and  therefore  upon  the  properties)  of  iron  and  steel  will  be  duly  considered 
in  these  pages. 

Influence  of  Thermal  Treatment.  —  The  size  of  the  crystalline  grains  of  iron  is 
affected  as  is  the  case  with  other  metals,  by  the  speed  of  its  solidification  and  subse- 
quent cooling,  slow  cooling  promoting  the  formation  of  large  grains.  When  the  iron 
is  produced  in  a  pasty  condition  as  in  the  puddling  process  and  the  charcoal  hearths, 
the  dimensions  of  its  grains  depend  essentially  upon  the  temperature  from  which 
the  metal  is  allowed  to  cool  undisturbed  by  mechanical  work. 


CHAPTER   V  — PURE   IRON  111 

As  explained  in  the  preceding  chapter,  it  seems  probable  that  pure  metals  do  not 
undergo  any  crystalline  growth  on  reheating  (annealing)  unless  they  have  been  pre- 
viously strained.  For  similar  reasons  we  may  doubt  the  occurrence  of  any  crystalline 
growth  on  annealing  chemically  pure  iron  in  the  complete  absence  of  strain.  The  be- 
havior of  electrolytic  iron,  however,  presently  to  be  described  appears  to  oppose  this 
view. 

The  possible  part  played,  during  the  annealing  operation,  as  described  in  the 
preceding  chapter,  by  the  thin  films  of  amorphous  cement  believed  by  some  to  be 
present  between  the  grains  applies  to  pure  iron  as  well  as  to  other  pure  metals. 

The  influence  of  heat  treatment  upon  the  structure  and  physical  properties  of 
commercial  irons  and  steels  will  be  dealt  with  at  length  in  these  pages. 

Crystallizing  Properties  of  Electrolytic  Iron.  — •  Stead  and  Carpenter  have  made 
the  important  discover}'  that  electrolytic  iron  of  very  great  purity  appears  to  possess 


vs    A    /»V    .    -  v   ,W      1 

•  .^.7    ' 


Fig.  127.  —  Thin  sheet  of  electrolytic  iron  a  portion 
of  which  was  heated  to  925  deg.  C.  for  one  minute 
and  the  whole  sheet  cooled  in  air.  Magnified  4 
diameters.  (Stead.) 


crystallizing  properties  radically  different  from  those  of  other  irons.  When  sheets  of 
electro-deposited  iron  not  exceeding  0.012  in.  thick  (preferably  not  over  0.01  in.)  are 
heated  above  the  upper  critical  point  of  iron  and  then  cooled  below  that  point, 
enormous  crystals  are  formed  in  three  seconds  after  cooling  below  the  critical  point. 
The  coarse  crystals  are  sometimes  "equi-axed"  and  sometimes  "radial,"  but  accord- 
ing to  their  discoverers  there  is  no  reason  for  thinking  that  they  are  constitutionally 
different.  This  remarkable  crystalline  phenomenon  is  well  illustrated  in  P^igure  127, 
which  represents,  under  a  magnification  of  only  four  diameters,  the  structure  of  a 
strip  of  electrolytic  iron  heated  to  between  920  and  930  deg.  for  one  minute  and  cooled 
in  air.  It  was  then  cleaned  with  hydrochloric  acid,  dipped  into  strong  nitric  acid, 
washed,  and  dried.  The  central  portion  where  the  crystals  are  small  represent  a  por- 
tion which  was  not  heated  above  the  upper  critical  point,  its  structure  remaining  un- 
altered. The  lower  portion  of  the  strip  is  composed  of  what  the  authors  called  "ra- 
dial" or  "columnar"  crystals  of  great  size,  while  in  the  upper  part  of  the  strip  the 
ordinary  polyhedral  or  "equi-axed"  crystals  have  formed.  The  sharp  boundaries 


112  CHAPTER   V  — PURE   IRON 

between  the  fine-grained  unaltered  metal  and  the  portions  having  assumed  a  coarse 
crystallization  should  be  noted. 

The  authors  write  that  once  the  coarse  crystals  are  formed  they  cannot  be  de- 
stroyed except  either  by  cold  mechanical  work,  or  by  heating  above  the  upper  crit- 
ical point  followed  by  quenching,  or  by  a  very  extended  heating  above  that  point 
followed  by  slow  cooling.  They  add  that  very  prolonged  heating  above  the  upper 
critical  point  followed  by  slow  cooling  does  not  result  in  the  formation  of  coarse 
crystals. 

Bearing  in  mind  that  above  its  upper  critical  point  iron  exists  in  the  gamma 
allotropic  form,  the  above  phenomenon  seems  to  indicate  that  on  cooling  through  its 
upper  point  the  small  crystals  of  gamma  iron  existing  above  it  are  converted  into  very 
large  crystals  of  beta  iron,  or  if  we  doubt  the  existence  of  beta  iron,  then  of  alpha  iron, 
which  is  the  view  held  by  Stead  and  Carpenter.  The  striking  features  of  this  extra- 
ordinary crystalline  transformation  are  (I)  the  fact  that  unless  the  sheets  be  very 
thin  (less  than  0.012  in.  thick)  the  growth  does  not  occur,  (2)  the  very  large  size  of  the 
resulting  crystalline  grains,  and  (3)  the  great  speed  with  which  they  form. 

Benedicks  believes  that  the  condition  required  for  the  occurrence  of  the  phenome- 
non is  extreme  purity  and  absence  of  mechanical  inclusions.  The  influence  of  the 
thickness  of  the  sheet  has  been  explained  on  the  ground  that  in  very  thin  sheets  the 
crystals  can  grow,  so  to  speak,  only  in  two  directions,  and  thus  appear  extremely 
large,  although  their  volume  may  be  relatively  small.  In  other  words  the  thinner 
the  sheet  the  larger  the  new  crystals  would  appear  to  be,  since  crystals  of  equal  vol- 
ume would  necessarily  occupy  a  larger  area  —  they  would  be  spread  over  a  greater 
surface. 

Influence  of  Impurities.  —  Commercial  iron  is  always  contaminated  by  the  pres- 
ence of  at  least  five  elements,  namely,  manganese,  silicon,  phosphorus,  sulphur,  and 
carbon,  generally  referred  to,  although  often  wrongly,  as  impurities.  The  important 
question  of  the  influence  of  these  substances  upon  the  structure  and,  therefore,  upon 
the  properties  of  iron  and  steel  will  be  fully  considered  in  another  chapter,  when  it 
will  be  shown  that  these  elements  form  definite  compounds  with  iron,  FeSi,  Fe3C, 
Fe3P,  or  with  each  other,  Mn3C,  MnS,  and  that  some  of  these  compounds,  FeSi, 
Fe3P,  are  retained  by  the  iron  in  solid  solution,  while  others,  Fe3C,  Mn3C,  MnS,  arc 
rejected  to  the  boundaries  of  the  crystalline  grains  or  along  other  crystallographic 
planes,  the  former  two  giving  rise  to  the  formation  of  eutectoid  mixtures.  This 
behavior  of  the  impurities  of  iron  and  steel  conforms  with  the  general  behavior  of 
impurities  described  in  Chapter  IV. 


CHAPTER  VI 

WROUGHT  IRON 

Wrought  iron  is  the  11:11110  given  To  commercial  iron  free  enough  from  carbon  and 
other  impurities  to  lie  malleable  when  such  metal  is  manufactured  through  the  re- 
duction of  iron  ores  or  the  refining  of  east  iron  at  a  temperature  so  low  that  it  is 
obtained  in  a  pasty  condition  and,  therefore,  mechanically  mixed  with  a  considerable 
amount  of  the  slag  formed  during  the  operation. 

When  the  refilling  treatment  is  conducted  at  a  temperature  sufficiently  high  to 
deliver  the  resulting  products  in  a  molten  condition,  the  refined  metal  which  is  then 
free  from  slag  is  called  steel.  Wrought  iron  and  steel  may  otherwise  have  identical 
chemical  composition,  although  usually  steel  contains  more  manganese  and  less 
silicon  than  wrought  iron,  often  more  carbon  and  less  phosphorus. 

Commercial  iron  which  is  not  malleable  is  called  cast  (pig)  iron.  The  modern 
method  of  producing  wrought  iron  consists  in  refining  cast  iron  in  a  non-regenerative 
reverberatory  furnace  (the  puddling  furnace),  while  the  refining  of  cast  iron  for  the 
production  of  steel  is  conducted  (1)  in  the  Bessemer  converter  or  (2)  in  a  regenerative 
reverberatory  furnace  (the  Siemens  open-hearth  furnace).  Cast  (pig)  iron  is  the  result 
of  smelting  iron  ore  in  blast-furnaces. 

Wrought  iron  is  sometimes  called  "puddle"  or  "puddled"  iron,  from  the  name  of 
the  furnace  in  which  it  is  now  generally  manufactured.  Wrought  iron  made  in  the 
old  charcoal  hearths  either  directly  from  the  ore  or  from  pig  iron  (Catalan,  Lancashire, 
or  Walloon  processes)  is  called  "charcoal  hearth"  or  "charcoal"  iron.  Wrought  iron 
produced  in  the  puddling  furnace  from  small  pieces  of  steel  scrap  is  known  as  "bush- 
eleil"  iron,  apparently  from  the  fact  that  the  small  pieces  of  steel  used  can  be  gath- 
ered in  bushel  baskets.  The  product  resulting  from  the  reheating  to  a  welding  heat 
and  subsequent  working  of  piles  (fagots  or  faggots)  of  pieces  of  wrought  iron  and  steel 
scrap  is  called  "fagoted"  iron,  also  spelled  "faggoted"  iron.  Knobbled  iron  is 
wrought  iron  produced  by  the  knobbling  process  which  resembles  the  South  Wales 
process  in  which  pig  iron  is  first  melted  down  in  a  coke  refinery,  considerable  silicon 
and  carbon  being  eliminated,  and  then  treated  in  a  charcoal  hearth  somewhat  as  in 
the  Lancashire  process. 

Puddled  iron  and  charcoal  hearth  irons  are  generally  freer  from  manganese  than 
fagoted  and  busheled  irons,  while  charcoal  hearth  iron  generally  contains  less  phos- 
phorus and  less  slag  than  puddled  iron.  In  England  wrought  iron  is  also  called 
malleable  iron. 

When  wrought  iron  contains  enough  carbon  to  bo  hardened  by  quenching  it  was 
at  one  time  called  wrought  steel,  but  this  expression  is  now  obsolete. 

In  the  puddling  process  the  pasty  lumps  of  iron  removed  from  the  furnace  at  the 
end  of  the  operation  are  called  "puddle  (puddled)  balls"  while  the  roughly  elongated 

113 


114 


CHAPTER  VI  — WROUGHT  IRON 


masses  resulting  from  the  squeezing  of  puddle  balls  are  sometimes  called  "blooms." 
The  blooms  are  rolled  into  "muck  bars,"  or  "puddle  (puddled)"  bars.  The  product  re- 
sulting from  the  cutting,  piling,  heating,  and  rolling  of  muck  bars  is  known  as  "refined 
iron,"  "refined  bar,"  "merchant  bar,"  "single  rolled  iron,"  "single  refined  iron," 
or  "No,  2  iron";  if  subjected  to  a  second  piling,  heating,  and  rerolling,  as  "double 
(doubly)  refined  iron,"  "double  rolled  iron,"  "No.  3  iron,"  "best  bar,"  "wire  iron," 
or  also  "refined  bar."  (H.  P.  Tiemann.) 

Chemical  Composition.  —  Wrought  iron  contains,  besides  an  appreciable  amount 
of  slag,  a  small  proportion  of  carbon  and  small  quantities  of  the  usual  impurities, 
manganese,  silicon,  phosphorus,  and  sulphur. 

Microstructure  of  Longitudinal  Section.  — •  Upon  being  withdrawn  from  the  pud- 
dling furnace,  the  white  hot,  pasty  balls  of  wrought  iron  are  subjected  to  vigorous 


Fig.  128.  —  Wrought  iron.  Longitudinal 
section.  Magnification  not  stated. 
(Longmuir.) 


Fig.  129.  —  Wrought  iron. 
Magnified  100  diameters, 
thor's  laboratory.) 


Longitudinal   section. 
(Boynton  in  the  au- 


forging  or  squeezing,  thus  expelling  a  large  amount  of  slag  and  firmly  welding  to- 
gether the  particles  of  iron.  Through  additional  heating  and  forging  or  rolling  the 
metallic  mass  is  converted  into  such  elongated  shapes  as  blooms,  billets,  bars,  etc. 
These  operations  so  affect  the  structure  as  to  impart  unlike  appearances  to  sections 
cut  longitudinally,  i.e.  in  the  direction  of  forging  or  rolling,  and  sections  cut  trans- 
versally,  i.e.  at  right  angles  to  that  direction.  The  microstructure  of  the  longitudinal 
section  of  a  wrought-iron  bar  is  shown  in  Figures  128  and  129.  From  our  knowledge 
of  the  chemical  composition  of  wrought  iron  we  should  be  able  to  anticipate  its 
microstructure.  The  ground  mass  or  matrix  of  the  metal  consists  of  polyhedral 
crystalline  grains  of  iron,  that  is  of  ferrite,  similar  in  every  respect  to  the  crystalline 
grains  of  pure  iron  and  of  pure  metals  in  general  described  in  Chapters  IV  and  V. 
The  ferrite  of  wrought  iron,  however,  as  explained  in  Chapter  V,  is  not  pure  iron 
but  rather  a  solid  solution  of  iron  in  which  are  dissolved  small  quantities  of  silicon, 


CHAPTER   VI  — WROUGHT   IRON  115 

phosphorus,  and  other  minor  impurities.  This  true  character  of  commercial  ferrite 
is  too  often  lost  sight  of  and  the  constituent  considered  as  pure  iron.  The  difference 
in  coloration  between  adjacent  grains  of  this  commercial  ferrite  should  be  noted, 
and  will  of  course  be  readily  understood  in  view  of  the  explanation  given  in  Chapter 
IV  to  account  for  this  phenomenon.  Many  irregular  black  lines,  varying  much  in 
thickness  and  length,  but  all  running  in  the  same  direction  are  clearly  seen.  These 
lines  indicate  the  location  of  the  slag  which  has  assumed  the  shape  of  fibers  or  streaks 
running  in  the  direction  of  the  rolling  or  forging,  thus  imparting. a  jibrous  appearance 
to  the  metal. 

The  presence  of  a  small  amount  of  carbon  in  wrought  iron  results  in  the  occurrence 
of  a  new  constituent  in  the  shape  of  small  dark  particles  located  between  some  of  the 
grains.  Under  low  magnification  these  carbon-holding  particles  are  not  readily  dis- 


-    »  ••• 


.-.    • . ..  •/*.  . 

*•-  *-/M 

".'*• 

Ifr  9  -."."'**'*"       ' 

§  - 


-»    •    .'• 


Fig.  130.  —  Wrought  iron.     Transverse  Fig.  131.  —  Wrought  iron.     Transverse  section, 

section.      Magnification    not   stated.  Magnification  100  diameters.     (Boylston.) 

(Longmuir.) 

tinguishable  from  slag  particles  and  as  this  carburized  constituent  is  not  a  very  im- 
portant one  in  the  case  of  wrought  iron  it  seems  advisable  to  postpone  its  description. 
Summing  up,  wrought  iron  consists  essentially  of  a  mass  of  ferrite  containing  many 
elongated  particles  of  slag. 

Microstructure  of  Transverse  Section.  — •  The  microstructure  of  the  transverse 
section  of  a  wrought-iron  bar  is  illustrated  in  Figures  130  and  131.  Like  the  structure 
of  the  longitudinal  section,  it  consists  of  a  polygonal  network,  indicating  that  the 
metal  is  made  up  of  polyhedral  crystalline  grains  of  ferrite.  The  slag,  however,  which 
in  the  longitudinal  section  occurred  as  fibers  running  in  a  direction  parallel  to  the 
rolling  or  forging,  here  assumes  the  shape  of  irregular,  dark  areas,  corresponding  to 
the  cross  sections  of  the  slag  fibers.  It  will  be  noted  that  in  both  the  longitudinal  and 
transverse  sections  the  ferrite  grains  are  equi-axed,  that  is,  they  show  no  sign  of  having 
been  elongated  in  the  direction  of  the  rolling.  It  was  thought  for  many  years  that 
wrought  iron  actually  had  a  fibrous  structure  and,  indeed,  the  number  of  persons 
still  holding  this  view  is  surprisingly  large.  Many  valuable  properties  were  attrib- 


.11(1  CHAPTKH   VI  — WROUGHT   IRON 

uted  to  puddled  iron  on  account  of  its  "fibrous  structure"  which  were  denied  to 
steel  because  of  its  "crystalline  structure."  The  microscope  has  summarily  dis- 
posed of  this  erroneous  belief  in  showing  that  the  ferrite  constituting  the  bulk  of 
wrought  iron  is  in  no  way  different  from  the  ferrite  forming  the  bulk  of  low  carbon 
steel.  Both  are  equally  crystalline. 

Structural  Differences  between  Various  Kinds  of  Wrought  Iron.  —  According  to 
W.  Campbell  charcoal  hearth  iron  can  sometimes  be  distinguished  from  puddled  iron 
through  the  presence  in  the  former  of  bands  of  ferrite  containing  more  or  less  pearlite 
(the  carburized  constituent  of  steel  soon  to  be  described)  especially  in  their  centers. 
The  occurrence  of  these  bands  is  ascribed  to  the  carburizing  of  the  iron  by  the  glowing 
charcoal  during  the  process  of  manufacture.  Fagoted  iron  also  contains  ferrite  and 
pcarlite  bands  from  the  steel  scrap  used  in  its  production  but  in  these  the  pearlite 


Kiji.  132.  —  Particle  of  slag  in  wrought,  iron. 
Magnified  200  diameters,     ((iiiillet.) 

does  not  decrease  from  center  to  outside,  sharp  lines  of  division  existing  between  tin- 
bands  and  the  ferrite  matrix  of  the  iron.  It  is  difficult  to  distinguish  between  the 
structure  of  busheled  iron  and  that  of  puddled  iron  on  the  one  hand  and  of  charcoal 
iron  on  the  other;  when  ferritc-pearlite  bands  occur  in  busheled  iron  they  resemble 
those  present  in  charcoal  iron,  while  in  the  absence  of  such  bands  the  structure  re- 
sembles that  of  puddled  iron. 

Chemical  Composition  of  Slag.  -  The  essential  chemical  constituents  of  the  slag 
produced  in  the  puddling  furnace  and  retained  in  part  by  the  iron  are  iron  oxides, 
both  ferric  (Fe2O.-i)  and  ferrous  (FeO),  oxide  of  manganese  (MnO),  silica  (SiO2),  and 
phosphoric  acid  (PaOr,).  Of  these  the  oxides  of  iron  and  manganese  are  basic  in  their 
chemical  affinity  while  silica  and  phosphoric  acid  are  acid.  These  bases  and  acids 
combine  with  each  other  to  form  neutral  compounds:  silicates  and  phosphates  of 
iron  and  manganesei 

Microstructure  of  Slag.  —  When  highly  magnified  (Fig.  132)  it  is  found  that  the 
slag  fibers  are  really  made  up  of  at  least  two  constituents,  a  dark  and  a  lighter  one, 
the  light  constituent  moreover  often  occurring  in  the  form  of  small  rounded  areas. 


CHAPTER   VI  — WROrdHT    1KOX  117 

We  are  naturally  led  to  speculate  us  to  the  nature  of  these  two  distinct  constituents 
of  the  slag,  and  in  view  of  our  knowledge  of  the  chemical  composition  of  slug  as  stated 
in  the  preceding  paragraph  we  are  tempted  to  conclude  that  one  of  the  constituents 
is  a  silicate  of  iron  and  manganese  while  the  other  is  a  phosphate  of  the  same  bases. 
The  accuracy  of  this  deduction,  however,  remains  to  be  proven. 

According  to  Matweieff  the  rounded  light  ureas  consist  of  iron  oxide  mixed  or 
not  with  manganese  oxide,  and  the  darker  background  of  silicate  of  iron  and  man- 
ganese. 

Matweieff  recommends  tho  following  method  to  distinguish  between  the  different  constituents 
of  slag.  The  polished  sample  placed  in  a  tube  is  heated  and  treated  by  a  current  of  pure  hydrogen 
which  causes  i  he  reduction  of  the  metallic  oxides  while  the  silicates  are  unaffected.  To  detect  the 
presence  of  ferrous  oxide  (FeO)  the  sample  heated  to  a  red  heat  is  acted  upon  by  steam,  a  treatment 
resulting  in  oxidizing  the  ferrous  oxide  into  magnet ie  oxide  (Fc3().i),  while  the  silicates  again  remain 
unaltered.  To  detect  the  presence  of  manganese  in  the  particles  of  oxides  revealed  by  the  hydrogen 
treatment  the  previously  treated  sample  is  repolished  and  etched  with  a  dilute  solution  of  ferric 
chloride  in  alcohol:  if  the  white  metallic  grains  resulting  from  tin-  hydrogen  treatment  are  colored 
darker  than  the  surrounding  iron  they  contain  some  manganese.  Finally  to  detect  the  presence  of 
iron  and  manganese  sulphide  the  polished  sample  is  etched  with  a  dilute  solution  of  tartaric  acid 
which  colors  sulphide  of  manganese  lightly  and  iron  sulphide  decidedly. 

Rosenhain  considers  it  probable  that  the  two  distinct  constituents  of  wrought- 
iron  slag  are  two  different  silicates  or,  possibly,  oxides  of  iron. 

It  is  seen  that  writers  generally  ignore  the  presence  of  phosphoric  acid  in  the  slag 
from  the  puddling  furnace,  and  still  it  generally  contains  from  3  to  5  per  cent  of  it, 
and  occasionally  considerably  more.  If  we  assume  that  this  phosphoric  acid  forms 
with  iron  a  phosphate  of  the  formula  SFeO.P-A,,  a  simple  calculation,  according  to 
atomic  weights  of  the  elements  involved,  will  show  that  the  presence  of  5  per  cent  of' 
phosphoric  acid  would  mean  the  formation  of  over  12.50  per  cent  of  this  phosphate 
of  iron.  It  is  hardly  to  be  supposed  that  this  phosphate  is  absorbed  by  some  other 
constituent  of  the  slag.  On  the  contrary  it  seems  highly  probable  that  it  must  be 
present  as  a  distinct  constituent. 

Influence  of  Thermal  and  Mechanical  Treatments.  —  The  dimensions  of  the  fer- 
rite  grains  of  wrought  iron  are  affected  by  the  treatments,  both  thermal  and  mechan- 
ical, received  by  the  metal.  The  effect  of  these  treatments  upon  the  structure  of 
iron  and  steel  will  be  considered  in  another  chapter. 


CHAPTER  VII 

LOW  CARBON  STEEL 

In  this  and  the  following  chapters  steel  will  be  considered  as  a  pure  alloy  of  iron 
and  carbon,  i.e.  free  from  the  impurities  (silicon,  manganese,  sulphur,  and  phosphorus) 
always  present  in  commercial  products.  The  influence  of  these  elements  upon  the 
structure  of  steel  will  form  the  subject  of  another  chapter. 

Normal  Structure.  —  The  structures  described  in  this  and  the  next  chapter  refer 
to  the  condition  of  steel  after  forging  followed  by  heating  to,  and  slowly  cooling  from, 
a  high  temperature  (900  to  1000  deg.  C.).  Such  treatments,  for  reasons  that  will 
be  understood  later,  promote  soundness,  remove  internal  strains,  prevent  excessive 
coarseness  of  structure  (as  in  castings) ,  and  permit  a  state  of  stable  equilibrium  to  be 
assumed  by  the  constituents.  The  resulting  structure  may  be  conveniently  called 
the  "normal"  structure  and  it  will  be  so  called  in  these  pages. 

Grading  of  Steel  vs.  Carbon  Content.  —  Steel  is  generally  graded  according  to 
the  amount  of  carbon  it  contains.  The  following  terms  are  those  most  commonly 
used: 

Very  low  carbon  steel,  very  mild,  or  extra  mild  steel, 

very  soft  or  dead  soft  steel carbon  not  over  0.10  per  cent 

Low  carbon  steel,  mild  steel,  soft  steel carbon  not  over  0.25  per  cent 

Medium  high  carbon  steel,  half  hard  steel    .    .    .  * .    .  carbon  0.26  to  0.60  per  cent 

High  carbon  steel,  hard  steel carbon  over  0.60  per  cent 

Very  high  carbon  steel,  very  hard,  or  extra  hard  steel  carbon  over  1.25  per  cent 

This  classification  is  somewhat  arbitrary  as  there  are  no  sharp  lines  of  demarcations 
universally  recognized  between  the  various  grades. 

It  will  be  seen  in  another  chapter  that  steel  containing  about  0.85  per  cent  carbon 
is  also  known  as  eutectoid  steel,  steel  containing  less  carbon  as  hypo-eutectoid  steel, 
and  more  highly  carburized  metal  as  hyper-eutectoid  steel. 

Low  Carbon  Steel  vs.  Wrought  Iron.  —  As  already  mentioned  the  distinction 
between  low  carbon  steel  and  wrought  iron  is  based  upon  the  difference  between  the 
methods  employed  for  their  respective  manufacture  rather  than  upon  unlike  chemical 
or  physical  properties,  for  these  metals  may  indeed  be  quite  identical  both  physically 
and  chemically.  The  mere  melting  of  wrought  iron  would  undoubtedly,  in  accord- 
ance with  the  universally  accepted  definition  of  steel  convert  it  into  steel  since  we 
would  now  have  a  malleable  metal  initially  cast.  Such  treatment  would  of  course 
result  in  the  elimination  of  the  slag  mechanically  retained  by  the  wrought  iron:  the 
melted  metal  would  be  slagless,  barring  cemented  steel,  another  essential  property  of 
steel.  Since  wrought  iron  generally  contains  but  a  small  amount  of  carbon,  melting 
it  would  convert  it  into  low  carbon  steel. 

118 


CHAPTER  VII  — LOW  CARBON  STEEL 


119 


The  Structure  of  Low  Carbon  Steel.  —  From  the  above  considerations  regarding 
the  resemblance  between  wrought  iron  and  low  carbon  steel,  the  structure  of  the 
latter  may  fairly  be  anticipated.  Seeing  that  low  carbon  steel  may  be  considered  as 
wrought  iron  from  which  the  mechanically  held  slag  has  been  expelled  through  melt- 
ing, we  should  expect  the  absence  of  slag  to  be  the  only  marked  difference  between 
the  structure  of  low  carbon  steel  and  that  of  wrought  iron. 

The  microstructure  of  low  carbon  steel  is  illustrated  in  Figures  133  to  138.  It  will 
be  seen  (Figs.  133  to  136)  to  consist  chiefly  of  a  mass  of  ferrite  (carbonless  iron)  ex- 
hibiting the  usual  polyhedral  crystalline  grains  described  in  preceding  chapters.  The 
ferrite  present  in  low  carbon  steel  is  similar  in  every  respect  to  the  ferrite  of  wrought 
iron.  At  the  junctions  of  many  ferrite  grains,  however,  some  dark  areas  will  be  noted, 


Fig.  133.  —  Steel.     Carbon  0.08  per  cent.          Fig.  134.  —  Steel.     Carbon  about  0.20  per  cent. 
Magnification  not  stated.     (Arnold.)  Magnified  200  diameters.     (Guillet.) 


an  evidence  of  the  existence  in  the  metal  of  another  constituent.  Since  ferrite  is  prac- 
tically free  from  carbon,  it  is  evident  that  the  carbon  present  in  the  steel  must  have 
segregated  into  these  small  dark  masses.  As  to  the  exact  nature  of  this  dark  constit- 
uent it  will  be  apparent  that  it  cannot  consist  of  pure  carbon  for  it  is  well  known  that 
the  carbon  present  in  steel  does  not  exist  in  the  free  state  but  on  the  contrary  is  com- 
bined with  some  of  the  iron  forming  a  definite  chemical  compound  or  carbide  of  iron 
whose  formula  is  Fe.iC.1  This  iron  carbide  must  necessarily  be  located  in  the  dark 
areas,  but  are  these  made  up  exclusively  of  this  carbide?  To  find  an  answer  to  this 
question  let  us  examine  the  structure  of  steel  under  a  higher  magnification  (Figs.  137 
and  138).  This  reveals  the  existence  of  two  components  in  each  dark  particle  occur- 
ring as  small  wavy  or  curved  parallel  plates  or  lamellae  alternately  dark  and  white. 
As  to  the  nature  of  these  two  components,  it  is  evident  that  one  of  them  must  be  the 

1  The  existence  of  the  carbide  FesC  in  unhardened  steel  was  first  shown  in  1885  by  Abel  and 
Muller,  working  independently,  and  has  since  been  confirmed  by  many  other  investigators.  Its 
existence  is  proved  by  dissolving  unhardened  steel  in  a  suitable  solvent  and  analyzing  the  carbona- 
ceous residue. 


120 


CHAPTKH    VII  — LOW    CAHBOX    STKKL 


carbide  Fe^C  and  the  other  necessarily  iron  or  ferrite,  since  according  to  the  proxi- 
mate analysis  of  steel,  these  are  the  only  two  constituents  which,  to  the  best  of  our 
knowledge,  are  present  in  pure  unhardened  carbon  steel. 

Pearlite.  —  Howe  named  the  microscopical  constituent  just  described  pearlite 
(originally  written  pearlyte)  following  in  this  Dr.  Sorby  who  was  the  first  observer  to 
describe  it  and  who  had  proposed  for  it  the  name  of  "pearly  constituent"  because  it 
frequently  exhibits  a  display  of  colors  very  suggestive  of  mother-of-pearl,  especially 
when  viewed  by  oblique  illumination.  This  appearance  is  due  to  the  fact  that  these 
plates  are  extremely  thin,  seldom  measuring  over  .» 3 » » »  of  an  inch  in  thickness,  and 
that  the  plates  of  carbide,  being  much  harder  than  the  ferrite  plates,  stand  in  relief 


V-j  •' 


, 


Fig.  13.5. — Steel.     Carbon  0.08  per  cent.      Magnified    100  diame- 
ters.     (Boylston.) 


after  polishing,  resulting  in  an  arrangement  very  similar  to  the  refraction  gratings  of 
physicists.  Mother-of-pearl  likewise  is  made  up  of  very  thin  alternate  plates  of  dif- 
ferent colors  and  possibly  of  different  hardness.  The  carbide  plates  remain  bright 
not  being  affected  by  the  usual  etching  reagents,  while  the  ferrite  plates  appear  dark 
because  of  their  being  somewhat  tarnished  by  the  etching  and  also  because,  being 
depressed  owing  to  their  greater  softness,  they  stand  in  the  shadow  of  the  carbide 
plates.  It  will  be  shown  in  another  chapter  that  in  many  series  of  alloys  of  two  metals 
the  alloy  of  lowest  melting-point  called  the  "eutectic"  alloy,  nearly  always  exhibits 
a  composite  structure  like  that  of  pearlite,  i.e.  made  up  of  parallel  plates  alternately 
of  one  and  the  other  constituents.  It  will  also  be  shown  that  in  spite  of  this  very 
great  structural  resemblance  pcai-litc  is  not  a  true  eutectic  alloy.  Howe  proposed 


CHAPTER   VII  — LOW   CARKOX   STKKL  121 

to  call  "eutectoid"  the  kind  of  mechanical  mixture  found  in  pearlite  and  this  most 
appropriate  term  has  been  universally  adopted. 

Because  of  the  minute  dimensions  of  the  lamella'  of  pearlite  a  high  magnifica- 
tion, generally  not  less  than  250  or  300  diameters,  is  required  for  its  resolution. 

It  should  be  stated  here  that  pearlite  does  not  always  assume  such  a  distinctly 
laminated  structure.  In  many  instances  its  structure  remains  ill  defined  or  has  a 
granular  rather  than  a  lamellar  appearance,  while  its  behavior  towards  the  etching 
reagents  likewise  varies.  It  will  be  shown  at  the  proper  time  that  this  is  due  to  the 
treatments  to  which  steel  may  be  subjected  and  that  the  exact  nature  of  these  ill- 
defined  forms  of  pearlite  (often  called  transition  constituents)  has  given  rise  to  a  large 
amount  of  discussion  and  has  been  the  object  of  many  investigations.  It  may  be 
assumed  for  the  present  that  any  pearlite  which  is  not  distinctly  lamellar  is  not  true 
pearlite. 

It  will  be  noted  that  in  Figure  134  the  pearlite  occupies  about  twice  the  area  cov- 
ered by  the  same  constituent  in  Figure  133.  We  infer  from  this  that  the  amount  of 


,  -.-•*.     O***^*  A     -_l   <* 

^J  ^"fc 

4  •'  -  V  ',- 

c"I   rfV"      '  -*•  ^r" 

vr.^^-f  ^    *      £Jf 

»,.  .  v     £> 


'$mi 

*  *  L*  *  i 


S** 


*; 


J»-' 

Fig.  13(5. — Steel.      Carbon  about  0.20  }icr  cent. 

pearlite  in  low  carbon  steel  at  least  increases  progressively  with  the  carbon  content. 
Doubling  the  amount  of  carbon  doubles  of  course  the  proportion  of  the  iron  carbide 
in  the  steel,  and  since  the  amount  of  pearlite  is  apparently  also  doubled  it  follows  that 
iron  carbide  and  ferrite  must  unite  with  each  other  in  fixed  ratio  to  form  pearlite,  in 
other  words  that  pearlite  always  contains  the  same  proportion  of  carbide  and  hence 
also  of  carbon.  The  accuracy  of  this  conclusion  will  soon  be  shown. 

Free  Ferrite.  -  To  distinguish  between  the  ferrite  included  in  pearlite  and  the 
ferrite  forming  the  balance  of  low  carbon  steel,  the  latter  is  sometimes  called  "free" 
ferrite,  "structurally  free"  ferrite,  "excess"  ferrite,  "massive"  ferrite,  "non-eutec- 
toid1'  or  " pro-eutectoid "  ferrite,  "surplus"  ferrite.  In  these  lessons  it  will  be  re- 
ferred to  as  free  ferrite  while  the  ferrite  forming  part  of  the  pearlite  will  be  called 
pearlite-ferrite.  Some  writers  refer  to  the  latter  as  eutectoid-ferrite. 

In  the  absence  of  any  conclusive  evidence  to  the  contrary,  it  is  natural  to  infer 
that  free  ferrite  and  pearlite-ferrite  are  identical,  that  is,  pure  iron  in  pure  steel  and 


122 


CHAPTER   VII  — LOW   CARBON   STEEL 


iron  holding  in  solution  small  quantities  of  silicon  and  phosphorus,  and  possibly  of 
other  impurities,  in  impure  (commercial)  steel.1 

Cementite.  —  The  name  of  cementite  has  been  given  by  Howe  to  the  carbide 
FesC  and  universally  adopted.    The  term  is  derived  from  "cement"  steel  (cementa- 


Fig.    137.  —  Steel.       Structure   of   pearlite. 
Magnified  1000  diameters.     (Osmond.) 


Fig.  138.  —Steel.  Hypo-euteetoid. 
Magnified  750  diameters.  Pearl- 
ite particles  and  surrounding 
ferrite.  (Goerens.) 


tion  steel,  blister  steel,  converted  steel)  which  being  generally  a  high  carbon  steel 
contains  a  great  deal  of  this  carbide,  that  is,  of  cementite. 

According  to  the  atomic  weights  of  iron  (56)  and  of  carbon  (12)  cementite  must 
contain 

12  X  100 
3  X  56  +  12  =  6'67  P 


The  carbon  present  in  cementite  is  frequently  referred  to  as  "cement"  carbon, 
occasionally  as  carbide  carbon,  to  distinguish  it  from  other  forms  of  carbon  found  in 
iron  and  steel  and  to  be  described  later  (hardening  carbon,  graphitic  carbon,  temper 
carbon,  etc.). 

Cementite  is  an  extremely  hard  substance,  being  in  fact  the  hardest  of  all  the 
constituents  occurring  in  iron  and  steel,  harder  even  than  hardened,  high  carbon 
steel.  Howe  states  that  it  is  harder  than  glass  and  nearly  as  brittle.  As  it  scratches 

1  This  has  been  doubted  by  some  writers,  however,  who  have  noted  that  the  ferrite  of  some  pearl- 
ites  was  more  readily  colored  on  etching  than  free  ferrite  and  they  saw  in  this  and  in  some  other 
evidences  an  indication  that  pearlite-ferrite  may  be  less  pure  than  free  ferrite.  Benedicks  for  in- 
stance believes,  or  at  least  believed  at  one  time,  that  the  pearlite-ferrite  of  some  steels  could  con- 
tain as  much  as  0.27  per  cent  of  carbon  dissolved  in  beta  iron,  whereas  free  ferrite  is  in  the  alpha 
condition.  This  carburized  and  allotropic  ferrite  Benedicks  called  "ferronite." 


CHAPTER  VII  — LOW  CARBON  STEEL  123 

feldspar  but  not  quartz  it  is  generally  assigned  to  rank  6  or  6.5  in  the  Mohs  scale  of 
hardness.1 

It  will  be  shown  later  that  when  steel  contains  an  appreciable  amount  of  manga- 
nese, as  is  nearly  always  the  case  in  commercial  products,  a  portion  at  least  of  this 
manganese  also  forms  a  carbide  Mn3C  and  that  this  carbide  unites  with  the  iron 
carbide  FeaC  to  form  cementite.  It  is  well  to  bear  in  mind,  therefore,  that  in  com- 
mercial steel  cementite  generally  contains  besides  Fe3C  varying  amounts  of  this  car- 
bide of  manganese.  As  the  atomic  weight  of  manganese  is  nearly  the  same  as  that  of 
iron,  55  compared  to  56,  it  so  happens  that  the  presence  of  manganese  in  cementite 
affects  but  very  little  its  carbon  content,  which  for  all  practical  purposes  may  be 
taken  as  6.67  regardless  of  the  amount  of  manganese  it  may  contain.  Cementite 
containing  much  manganese  has  been  called  manganiferous  cementite  by  some 
writers. 

Recent  investigations  tend  to  show  that  cementite  may  also  contain  small  amounts 
of  silicon  and  sulphur  dissolved  in  it  possibly  as  silicide  and  sulphide  of  iron  respec- 
tively, although  Stead  and  W.  H.  Hatfield  mentioned  the  presence  of  silicon  carbide. 
It  will  be  shown  elsewhere  that  some  of  these  elements  (manganese  and  sulphur)  ap- 
parently increase  the  stability  of  cementite  while  silicon  decreases  it. 

Baykoff  has  advanced  the  theory  that  cementite  is  not  a  definite  compound  or 
mixture  of  definite  compounds  but  a  solid  solution. 

Cementite  generally  remains  bright  and  brilliant  after  the  ordinary  etching  treat- 
ments employed  to  reveal  the  structure  of  steel.  Sodium  picrate,  however,  colors 
it  deeply  (see  Chapter  VIII). 

1  The  Mohs  scale  is  as  follows,  beginning  with  the  softest  and  ending  with  the  hardest  mineral 
and  each  mineral  being  capable  of  scratching  the  preceding  ones:  (1)  Talc,  (2)  Gypsum,  (3)  Calcite, 
(4)  Fluorite,  (5)  Apatite,  (6)  Feldspar,  (7)  Quartz,  (8)  Topaz,  (9)  Corundum,  and  (10)  Diamond. 


CHAPTER  VIII 


.MEDIUM   HIGH   AND   HIGH   CARBON   STEEL 


Medium  High  Carbon  Steel.  —  The  normal  structure  of  steel  (i.e.  its  structure 
.after  working,  reheating  to  a  high  temperature  and  slow  cooling)  containing  about 
0.30  per  cent  carbon  is  illustrated  by  a  drawing  in  Figure  139  and  by  a  photomicro- 
graph in  Figure  140.  It  will  be  noted,  on  comparing  this  structure  with  that  of  lower 
carbon  steels  (Chapter  VII),  that  the  introduction  of  more  carbon  in  the  iron  has  re- 
sulted, as  would  be  expected,  in  the  occurrence  of  a  greater  amount  of  pearlite  and  of 
a  correspondingly  smaller  proportion  of  ferrite.  The  pearlite  occupies  now  roughly 
about  one  third  of  the  total  area.  The  junction  lines  between  the  grains  of  ferrite 


Fig.  139.  —  Steel.     Carbon  0.38  per  cent . 
Magnification  not  stated.     (Arnold.) 


Fig.  140. —  Steel.  Carbon  0.30  per  rent.  Mag- 
nified 1 50  diameters.  Forged  and  annealed. 
(C.  C.  Buck,  Correspondence  Course  student.) 


should  be  noted.    Under  sufficiently  high  power  the  pearlite  areas  exhibit  the  char- 
acteristic lamellar  structure  described  in  Chapter  VII. 

On  further  addition  of  carbon,  the  amount  of  pearlite,  which  is  evidently  propor- 
tional to  the  percentage  of  carbon,  increases  correspondingly,  as  shown  in  Figures  141 
and  142  illustrating  the  microstructure  of  steel  containing  about  0.50  per  cent  car- 
bon. The  pearlite  occupies  here  over  one  half  of  the  total  area.  It  will  be  noticed 
that  the  ferrite  areas  are  only  occasionally  resolved  into  polyhedral  grains,  appar- 
ently because  the  ferrite  now  occurs  in  particles  often  too  small  to  be  made  up  of 
several  crystalline  grains.  These  small  masses  of  ferrite,  however,  are  still  made  up 

124 


CHAI'TKH    VIII  — MKDIOI    HIGH    AND    HKiH    CARBON    STKKI,  125 


Fig.  141.  —  Steel.     Carbon  ()./><>  per  cent. 
Magnification  not  stated.     (Arnold.) 


Fig.  142.  —  Steel.  Carbon  ()..")()  per  cent. 
Magnified  100  diameters.  Heated  to  1000 
deg.  C.  and  slowly  cooled  in  furnace. 
(Burger.) 


Fig.  It:;.       Sled.     Carbon  0.4.">  per  cent.     Magnified 
1000  diameters.     (Osmond.) 


126  CHAPTER  VIII  — MEDIUM  HIGH  AND  HIGH  CARBON  STEEL 

of  crystalline  matter  as  described  and  illustrated  in  Chapter  V.  A  high-power  photo- 
micrograph of  0.45  per  cent  carbon  steel  is  shown  in  Figure  143.  The  laminations  of 
pearlite  are  clearly  seen. 

When  steel  contains  but  a  small,  although  appreciable,  amount  of  ferrite,  as  is 
the  case  with  carbon  contents  between  0.50  and  0.70  per  cent  the  ferrite  frequently 
forms  envelopes  or  membranes  surrounding  the  pearlite  grains,  an  arrangement 
generally  described  as  a  network  structure  the  pearlite  forming  the  meshes  and  the 
free  ferrite  the  net  proper.  These  pearlite  meshes  are  also  described  as  "cells"  or 
"kernels"  and  the  ferrite  membranes  as  "cell  walls"  or  "shells." 

It  will  be  shown  later  that  this  network  structure  is  promoted  by  rather  rapid 
cooling  from  a  high  temperature,  as  for  instance  by  cooling  small  pieces  in  air. 

Structures  of  this  type  are  illustrated  in  Figures  144  and  145.    The  latter  illustra- 


Fig.  144.  —  Steel.  Carbon  0.59  per  cent.  Magni- 
fied 100  diameters.  Heated  to  1000  deg.  C.  and 
cooled  in  air.  (Burger.) 

tion  is  of  special  interest  being  a  reproduction  of  one  of  Sorby's  original  drawings  and, 
therefore,  the  first  drawing  of  pearlite  ever  published. 

High  Carbon  Steel.  —  Since  the  introduction  of  increasing  amounts  of  carbon  in 
steel  results  in  the  formation  of  a  correspondingly  increasing  proportion  of  pearlite 
and  decreasing  proportion  of  ferrite,  a  degree  of  carburization  must  necessarily  be 
reached,  when  the  whole  mass  will  be  made  up  of  pearlite,  the  ferrite  having  finally 
disappeared.  This  critical  point  in  the  structure  of  steel  is  attained  when  the  metal 
contains  somewhere  between  0.80  and  0.90  per  cent  of  carbon,  exceptionally  pure 
steel  requiring  the  larger  proportion  of  carbon  and  impure  steel  the  smaller  for  the 
complete  disappearance  of  ferrite. 

Eutectoid  Steel.  —  Steel  made  up  exclusively  of  pearlite  is  now  quite  universally 
called  "eutectoid"  steel,  after  Howe,  the  name  suggesting  the  great  resemblance 
between  pearlite  and  eutectic  alloys  while,  at  the  same  time,  clearly  indicating  that 


CHAPTER  VIII  — MEDIUM   HIGH  AND   HIGH   CARBON   STEEL 


127 


pearlite  is  not  a  real  eutectic  alloy.  Previous  to  Howe's  happy  suggestion  this  steel 
was  commonly  described  as  "eutectic"  or  "saturated"  steel.  It  has  also  been 
termed  "aeolic"  or  "benmutic"  steel  but  these  names  have  now  been  abandoned. 
The  structure  of  eutectoid  steel  is  illustrated  in  Figures  146  and  147. 


Fig.  145.  —  Steel.     Hypo-eutectoid. 
(Sorby.) 


Fig.  146.  —  Steel.     Eutectoid.     Magnified 
410  diameters. 


.'////, 


Fig.  147.  —  Steel.     Eutectoid.     Magnified  1000  diameters.     (J.  V.  Emmons.) 

Steel  containing  less  than  0.85  per  cent  carbon  or  thereabout,  and  in  which, 
therefore,  some  free  ferrite  is  present,  is  called  "hypo-eutectoid,"  while  steel  more 
highly  carburized  than  eutectoid  steel  is  called  "hyper-eutectoid."  It  will  be  shown 
presently  that  hyper-eutectoid  steel  contains  free  cementite. 

Hyper-Eutectoid  Steel. --The  normal  structure  of  steel  containing  from  1.10  to 
1.50  per  cent  carbon  is  illustrated  in  Figures  148  to  151  both  under  low  and  high 


128 


CHAPTER    VIII  — MKIMTM    HIGH    AND    HIGH   CARBON   STKKL 


magnification.  These  steels  will  be  seen  to  consist,  like  hypo-eutectoid  steel,  of  two 
constituents,  one  of  which  is  pearlite  as  clearly  shown  when  examined  under  high 
power.  The  other  constituent  remains  bright  after  etching  and  might  at  first  be 
taken  for  ferrite.  Upon  reflection,  however,  it  will  be  evident  that  such  cannot  be  its 


Fig.  148.  —  Steel.     Carbon  1 .20  per  cent. 
Magnification  not  stated.     (Arnold.) 


Fig.  149.  —  St<vl. 


1.10  ])Cr 
i  Movnton. ) 


Magnified  100  diamet 


nature.  The  light  constituent  of  hyper-eutectoid  steel  consists  of  cementite  which 
is  now  present  in  excess  over  the  amount  required  to  form  pearlite,  just  as  in  hypo- 
eutectoid  steel,  ferrite  is  the  excess  constituent.  It  will  be  evident  that  the  ferrite 
and  cementite  which  constitute  all  grades  of  carbon  steels  combine  with  each  other 


CHAPTER  VIII  —  MEDIUM   HIGH   AND   HIGH   CARBON   STEEL  129 

in  suitable  proportions  to  form  pearlite,  leaving,  as  the  case  may  be,  an  excess  of 
ferrite  (in  hypo-eutectoid  steel)  or  of  cementite  (in  hyper-eutectoid  steel).  A  more 
scientific  explanation  of  the  formation  of  the  normal  structure  of  steel  will  be  offered 
in  a  subsequent  chapter. 

Free  Cementite.  —  To  distinguish  between  the  cementite  forming  part  of  the 
pearlite  (the  bright  plates  of  that  constituent)  and  the  cementite  constituting  the 
balance  of  hyper-eutectoid  steel,  the  latter  is  generally  called  "free"  cementite, 
"structurally  free"  cementite,  "excess"  cementite,  " massive  1  cementite,  "non- 
eutectoid"  cementite,  "surplus"  cementite,  while  the  cementite  included  in  the 


Fig.  150.  —  Steel.     Carbon  1.43  per  cent.     Magnified  50  diameters. 
(Boynton.) 

pearlite  is  sometimes  referred  to  as  pearlite-cementite  or  eutectoid  cementite.  In 
these  pages  the  cementite  in  excess  over  the  eutectoid  ration  will  be  called  free 
cementite. 

In  the  absence  of  any  conclusive  evidences  to  the  contrary,  and  in  conformity 
with  the  nature  of  eutectic  alloys  in  general,  it  is  assumed  that  free  cementite  and 
pearlite-cementite  are  identical  in  composition  and  properties. 

As  already  stated  in  Chapter  VII  the  cementite  of  commercial  steel  is  not  pure 
Fe3C  but  contains  small  and  varying  amounts  of  Mn3C,  and  possibly  also  a  little 
silicon  and  sulphur. 

As  shown  in  Figures  148  and  149  hyper-eutectoid  steel  like  hypo-eutectoid  steel 
may  assume  a  network  structure.  In  both  cases  the  meshes  consist  of  pearlite  but 
the  net  proper  which  in  hypo-eutectoid  steel  represents  membranes  of  free  ferrite 
indicates  now  the  occurrence  of  membranes  of  free  cementite. 

Hypo-  vs.  Hyper-Eutectoid  Steel.  —  While  there  is  considerable  similarity  be- 
tween the  structure  of  steel  containing  but  a  slight  excess  of  ferrite  and  the  structure 
of  steel  containing  but  a  slight  excess  of  cementite,  a  little  experience  and  careful 


130 


CHAPTER  VIII  — MEDIUM   HIGH   AND  HIGH   CARBON  STEEL 


examination  will  reveal  differences  in  their  appearances  and  properties  which  will 
make  it  possible,  generally,  to  distinguish  between  them.  Cementite  has  a  more 
metallic  luster  than  ferrite  and  remains  bright  and  structureless,  even  after  prolonged 
etching  with  the  ordinary  reagents,  while  ferrite  is  colored  and,  if  present  in  suf- 
ficiently large  masses,  resolved  into  grains  by  such  treatment.  Cementite  is  extremely 
hard,  standing  in  relief,  while  ferrite  being  soft  is  depressed  by  the  polishing  oper- 
ation. Ferrite  is  readily  scratched  by  a  needle  drawn  across  the  polished  surface  while 
Cementite  remains  unmarked.  Again  it  will  be  noted  that  in  Figures  148  and  150 
some  of  the  pearlite  grains  are  cut  by  plates  or  needles  of  cementite,  independent  of 
the  network  of  cementite,  an  occurrence  seldom  observed  when  ferrite  is  the  excess 
constituent. 

When  cementite  is  in  excess  the  pearlite  grains  are  generally  smaller  and  the  net- 
work finer  than  is  the  case  with  excess  of  ferrite.    Free  cementite  does  not,  of  course, 


Fig.  151.  — Steel. 


Carbon  1.43  per  cent. 
(Boynton.) 


Magnified  500  diameters. 


always  assume  the  shape  of  a  fine  network.  It  will  be  shown  in  subsequent  chapters 
that  its  mode  of  occurrence  depends  upon  the  treatment  to  which  the  steel  was  sub- 
jected. 

Etching  of  Cementite.  —  It  has  been  seen  that  cementite  is  not  acted  upon  by  the 
usual  reagents  employed  in  the  etching  of  steel  sections  (picric  acid,  nitric  acid,  tinc- 
ture of  iodine,  etc.)  but  that  it  is  darkly  colored  by  a  suitable  treatment  with  sodium 
picrate  (Chapter  II)  while  pearlite  on  the  contrary  remains  bright. 

In  Figures  152  and  153  is  shown  the  structure  of  the  same  specimen  of  hyper- 
eutectoid  steel  etched  with  nitric  acid  (Fig.  152)  and  with  sodium  picrate  (Fig.  153). 

Carbon  Content  of  Pearlite.  —  The  percentage  of  carbon  in  pearlite  and,  there- 
fore, in  eutectoid  steel,  has  been  stated  to  be  somewhere  between  0.80  and  0.90  in 
commercial  steel  of  ordinary  quality,  because  when  steel  of  that  degree  of  carburiza- 
tion  is  examined  under  the  microscope  it  is  found  to  be  free  from  any  appreciable 


CHAPTER   VIII  — MEDIUM   HIGH   AND   HIGH   CARBON   STEEL 


131 


amount  of  free  ferritc  or  of  free  cementitc.  As  the  presence  of  a  very  small  amount 
of  any  of  these  two  constituents  in  the  free  state,  however,  is  very  difficult  to  ascer- 
tain, it  will  be  evident  that  it  is  quite  impossible  to  speak  positively  as  to  the  exact 


Fig.  152.  —  Steel.  Hyppr-cutectoid.  Etched  with  nitric  acid.  Cem- 
entite  uneolored,  pcarlite  colored.  Magnified  100  diameters.  (R.  W. 
Smyth  in  the  author's  laboratory.) 


Fig.  153.  —  Same  as  Fig.  152.  Etched  with  sodium  picrate.  Cem- 
entitc colored  dark,  pearlite  uncolored.  (R.  W.  Smyth  in  the 
author's  laboratory.) 


amount  of  carbon  needed  to  exclude  both  free  ferrite  and  free  cementite  from  the 
structure.  Moreover  this  carbon  content  of  pearlite  varies  somewhat  with  the  com- 
position of  the  steel  and  with  the  treatment  it  has  received.  In  steel  of  ordinary 


132  CHAPTER  VIII  —  MEDIUM   HIGH  AND  HIGH  CARBON  STEEL 

commercial  purity  the  eutectoid  point  appears  to  be  in  the  vicinity  of  0.85  per  cent 
carbon. 

Structural  Composition  of  Steel.  —  Bearing  in  mind  that  hypo-eutectoid  steel  is 
composed  of  free  ferrite  and  pearlite  and  that  hyper-eutectoid  steel  consists  of  free 
cementite  and  pearlite,  and  knowing  the  proportion  of  carbon  in  pearlite  (0.85  per 
cent?)  and  in  cementite  (6.67  per  cent),  the  structural  composition  of  any  steel  may 
be  readily  calculated,  provided  we  know  the  percentage  of  carbon  it  contains. 

In  case  of  hypo-eutectoid  steel  we  have  the  two  following  equations: 

(1)  F  +  P  =  100 


in  which  F  represents  the  percentage  of  free  ferrite  in  the  steel,  P  the  percentage  of 
pearlite,  E  the  percentage  of  carbon  in  pearlite,  and  C  the  percentage  of  carbon  in 
the  steel.  The  first  equation  expresses  the  fact  that  the  steel  is  composed  of  ferrite 
and  pearlite  and  the  second  equation  the  fact  that  all  the  carbon  in  the  steel  is  in- 
cluded in  the  pearlite.  Assuming,  for  instance,  that  pearlite  contains  0.85  per  cent 
carbon  and  the  steel  0.50  per  cent  carbon,  the  resolution  of  these  two  equations 
indicates  that  steel  of  that  grade  has  the  following  structural  composition: 

F  =  per  cent  free  ferrite  =  41.18 
P  =  per  cent  pearlite  =  58.82 

In  case  of  hyper-eutectoid  steel  the  following  two  equations  may  be  written: 

(1)  P  +  Cm  =  100 


in  which  P  represents  the  percentage  of  pearlite,  Cm  the  percentage  of  free  cemen- 
tite, E  the  percentage  of  carbon  in  pearlite,  C  the  percentage  of  carbon  in  the  steel. 
The  first  equation  expresses  the  fact  that  hyper-eutectoid  steel  is  composed  of  pearlite 
and  free  cementite  and  the  second  the  fact  that  the  carbon  in  the  steel  is  distributed 
between  the  pearlite  and  the  free  cementite,  forming  E  per  cent  of  the  pearlite  and 
6.67  per  cent  of  the  cementite.  Assuming  the  value  of  E  to  be  0.85  and  the  steel 
to  contain  1.25  per  cent  carbon,  these  equations  give  for  a  steel  of  that  grade 

P  =  per  cent  pearlite  =  93.13 
Cm  =  per  cent  free  cementite  =  6.87 

Supposing  that  pearlite  or  eutectoid  steel  contains  0.85  per  cent  carbon,  since  the 
whole  of  that  carbon  is  present  in  the  cementite  plates  of  pearlite  and  since  cemen- 
tite contains  6.67  per  cent  carbon  (as  called  for  by  its  chemical  formula  Fe3C),  the 
percentage  of  cementite  in  pearlite  may  be  readily  calculated,  as  follows  : 

X  per  cent  cementite  =  0.85 

100 

hence,  per  cent  cementite  =  0.85  X  -    „  =  12.74 

6.o/ 

and  per  cent  ferrite  =  100  -  12.74  =  87.26 
or  roughly  1  part  by  weight  of  cementite  to  6.6  parts  by  weight  of  ferrite. 


CHAPTER   VIII  — MEDIUM   HIGH  AND   HIGH   CARBON   STEEL  133 

If  it  be  considered,  however,  (1)  that  the  exact  carbon  content  of  pearlite  is  not, 
and  hardly  can  be,  known,  (2)  that  it  varies  somewhat  both  with  composition  and 
treatment,  and  (3)  that  in  commercial  steel  it  is  probably  not  far  from  0.85  per  cent, 
we  are  fully  warranted  to  assume,  for  the  sake  of  the  great  simplicity  it  introduces 
in  the  calculations,  that  pearlite  contains  exactly  1  part  by  weight  of  cementite  to 
7  parts  by  weight  of  ferrite,  which  would  be  the  case  if  the  eutectoid  point  corre- 
sponded to  0.834  per  cent  carbon,  as  indicated  below: 

1  part  cementite  +  7  parts  ferrite  yields  8  parts  pearlite 
or  12.50  per  cent  cementite  +  87.50  per  cent  ferrite  =  100  per  cent  pearlite 

and  since  cementite  contains  6.67  per  cent  carbon,  12.50  per  cent  cementite  will  con- 
tain 6.67  X  .1250  =  0.834  per  cent  carbon.  Assuming  then  that  such  is  the  carbon 
content  of  eutectoid  steel,  so  that  1  part  of  cementite  gives  exactly  8  parts  by  weight 
of  pearlite  and,  noting  that  the  carbon  in  the  steel  produces  exactly  15  times  its  own 
weight  of  cementite,1  the  calculation  of  the  structural  composition  of  any  steel  be- 
comes extremely  simple. 

In  case  of  hypo-eutectoid  steel  (steel  containing  less  than  0.834  per  cent  carbon) 
we  have, 

per  cent  total  cementite  =  per  cent  total  carbon  X  15 
and  per  cent  pearlite  =  per  cent  total  cementite  X  8 

or,  more  simply, 

per  cent  pearlite  =  per  cent  carbon  X  120 
i.e.  P  =  120  C 

and,  of  course,  per  cent  ferrite  =  F  =  100  —  P. 

With  hyper-eutectoid  steel  (steel  containing  more  than  0.834  per  cent  carbon)  the 
figuring  is  as  follows: 

Since  8  parts  of  pearlite  contain  7  parts  of  ferrite  and  since  in  hyper-eutectoid 
steel  the  totality  of  the  ferrite  (total  ferrite)  is  included  in  the  pearlite  (there  being 
no  free  ferrite)  we  have 

Total  ferrite  =  F  =  %  pearlite  (P) 
or  per  cent  pearlite  =  P  =  ?  total  ferrite 
and,  since  total  ferrite  =  100  —  total  cementite, 
P  =  ?  (100  -  total  cementite) 

But  total  cementite  =  carbon  X  15,  hence 

P  =  ?  (100  -  15  C) 

800  -  120  C 
or  P  =        

or  approximately  P  =  114  —  17  C 
and,  of  course,  free  cementite  =  Cm  =  100  —  P. 

Summing  up,  in  order  to  find  the  percentage  of  pearlite  in  hypo-eutectoid  steel  it 
will  suffice  to  multiply  its  carbon  content  by  120  (P  =  120  C),  the  balance  of  the  steel, 

1  This  follows  from  the  composition  of  Fe3C  indicated  by  the  atomic  weights  of  iron  and  carbon : 

(3  x  56)  iron  +  12  carbon  =180  Fe3C 

180 
hence,  1  part  carbon  produces  -.—  =  15  parts  Fe3C  or  cementite. 


134  CHAPTER  VIII  — MEDIUM   HIGH  AND  HIGH  CARBON  STEEL 

consisting,  of  course,  of  free  ferrite  (F  =  100  —  P);  to  find  the  percentage  of  pearlite 
in  hyper-eutectoid  steel,  the  percentage  of  carbon  in  the  steel  should  be  substituted 

OQQ    1OQ    f] 

for  C  in  the  formula:  P  =  (or  approximately  P  =  114  —  17  C)  and 

the  balance  of  the  steel  will  be  made  up  of  free  cementite  (Cm  =  100  —  P). 

Taking,  for  instance,  a  steel  containing  0.50  per  cent  carbon,  its  structural  com- 
position will  be: 

120  X  0.50  =  GO  per  cent  pearlite,  and 

100  —  60  =  40  per  cent  ferrite 

If  a  steel  contains  1.25  per  cent  carbon  the  resulting  percentage  of  pearlite  will 

800  -  120  X  1.25  , 

be  -         — ~ —  (or  more  simply  but  less  accurately  114  --  17  X  1.25)  or  re- 

spectively 92.86  and  92.75  according  to  the  formula  used.  The  balance  is  free  cementite. 
In  these  pages  it  will  be  assumed  for  the  sake  of  the  simplicity  it  introduces  that 
pearlite  contains  0.834  per  cent  carbon,  that  is  exactly  1  part  by  weight  of  cementite 
to  7  parts  of  ferrite. 

Chemical  vs.  Structural  Composition.  —  Disregarding  for  the  present  the  existence 
of  impurities,  the  ultimate  analysis  of  steel  reveals  the  presence  of  so  much  carbon 
and  so  much  iron.  The  proximate  chemical  analysis  of  steel  reveals  (in  steel  slowly 
cooled  from  a  high  temperature)  the  presence  of  so  much  iron  and  so  much  carbide 
of  iron,  FeaC.  In  a  similar  way  we  may  consider  two  different  structural  composi- 
tions, an  ultimate  and  a  proximate  one.  The  ultimate  structural  composition  reveals 
the  presence  of  so  much  total  ferrite  and  so  much  total  cementite,  while  the  proxi- 
mate structural  composition  informs  us  of  the  percentages  of  pearlite,  free  ferrite, 
and  free  cementite  in  the  steel.  It  will  be  evident  that  the  chemical  proximate  com- 
position is  identical  to  the  ultimate  structural  composition,  the  names  of  the  con- 
stituents only  being  different,  iron  and  carbide  in  the  first  case,  ferrite  and  cementite 
in  the  latter. 

These  various  compositions  are  tabulated  below: 

Constituents 

ultimate        Fe  C 

proximate     Fe  Fe3C 

ultimate        total  ferrite    total  cementite 
proximate     pearlite  free  ferrite  free  cementite 


Chemical  Composition 
Structural  Composition 


It  is  apparent  that  the  proximate  structural  composition  affords  more  valuable 
information  than  is  obtainable  through  the  other  three  kinds  of  analysis,  for  not  only 
does  it  indicate  the  chemical  nature  of  the  proximate  constituents  but  also  their 
structural  association  and  occurrence,  upon  which  depend,  to  a  very  great  extent,  the 
physical  properties  of  steel.  In  the  following  table  the  ultimate  chemical  composi- 
tion as  well  as  the  structural  composition,  both  ultimate  and  proximate,  of  steel 
containing  from  0.1  to  2.0  per  cent  of  carbon,  have  been  calculated  for  each  in- 
crease of  carbon  of  0.1  per  cent.  Corrections  for  variations  of  0.01  per  cent  carbon 
can  readily  be  obtained  by  interpolation  and  are  indicated  in  a  second  table.  The 
values  given  for  the  proximate  compositions  are  based  upon  the  assumption  that 
pearlite  contains  0.834  per  cent  carbon. 


CHAPTER   V11I  —  MEDIUM   HIGH   AND   HIGH   CARBON   STEEL 


135 


CHEMICAL  COMPOSITION 

STRUCTURAL  COMPOSITION 

ULTIMATE 

ULTIMATE 

PROXIMATE 

C 

Fe 

Total  Cementite 

Total  Ferrite 

Pearlite 

Free  Ferrite 

Free  Cementite 

.1 

99.9 

1.8 

98.5 

12.0 

88.0 



.2 

99.8 

3.0 

97.0 

24.0                  76.0 

— 

.3 

99.7 

4.5 

95.5 

36.0 

-64.-0 

— 

.4 

99.6 

0.0 

94.0 

48.0                  52.0 

— 

.5 

99.5 

.     7.5 

92.5 

60.0                  40.0 

— 

.6 

99.4 

9.0 

91.0 

72.0                  28.0 

— 

.7 

99.3 

10.5 

89.5 

84.0                  16.0 

— 

.8 

99.2 

12.0 

88.0 

96.0                    4.0 

— 

.9 

99.1 

13.5 

86.5 

98.7 

1.3 

1.0 

99.0 

15.0 

85.0 

97.0 

3.0 

1.1 

98.9 

16.5 

83.5 

95.3 

4.7 

1.2 

98.8 

18.0 

82.0 

93.6 

— 

6.4 

1.3 

98.7 

19.5 

80.5 

91.9 

8.1 

1.4 

98.6 

21.0 

79.0 

90.2 

9.8 

1.5 

98.5 

22.o 

77.5 

88.5 

11.5 

1.8 

98.4 

24.0 

76.0 

86.8 

— 

13.2 

1.7 

98.3 

25.5 

74.5 

85.1 

— 

14.9 

1.8 

98.2 

27.0 

73.0 

83.4 

— 

16.6 

1.9 

98.1 

28.5 

71.5 

81.7 

— 

18.3 

2.0 

98.0 

30.0 

70.0 

80.0 

— 

20.0 

1 

1 

CARBON 

Hvpo-EuTECTOiD  STEEL 

HYPER-EUTECTOID    STEEL 

« 

Values  to  be  added  to  %  of  peariite  and 
subtracted  from  %  ferrite 

Values  to  be  subtracted  from  %  pearlite  and 
added  to  %  cementite 

0.01 

1.2 

0.17 

0.02 

2.4 

0:34 

0.03 

3.6 

0.51 

0.04 

4.8 

0.68 

0.0.5                                                6.0 

0.85 

0.06                                                7.2 

1  .02 

0.07 

8.4 

1.19 

0.08 

9.0 

1.36 

0.09 

10.8 

1.53 

These  compositions  are  shown  also  cliagrammatically  in  Figure  154  which  will  be 
readily  understood.  ABC  represents  the  free  ferrite  in  hypo-eutectoid  steel,  ACD  the 
pearlite  in  hypo-eutectoid  steel,  DCEF  the  pearlite  in  hyper-eutectoid  steel,  DFG  the 
free  cementite  in  hyper-eutectoid  steel,  ABEH  the  total  ferrite  in  any  steel,  AHG 
the  total  cementite  in  any  steel,  ACEH  the  pearlite-ferrite  in  any  steel,  and  AHFD 
the  pearlite-cementite  in  any  steel. 

Micro-Test  for  Determination  of  Carbon  in  Steel.  —  Since  the  amount  of  pearlite 
in  steel  is  proportional  to  the  percentage  of  carbon  it  contains,  it  should  be  possible 
to  estimate  the  latter  with  a  fair  degree  of  accuracy  from  the  area  occupied  by  the 


136 


CHAPTER   VIII  — MEDIUM    HIGH   AND   HIGH   CARBON   STEEL 


ly 


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cvi  »} 


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N    <\J 


*  M- 
v  ft 


o 

p. 


*     C\J 


o 

g 


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to 


CHAPTER   VIII  — MEDIUM   HIGH   AND   HIGH   CARBON   STEEL  137 

pearlite.  After  a  little  experience  and  by  taking  the  necessary  precautions  it  will  be 
found  that,  in  the  case  of  decidedly  hypo-eutectoid  steels  at  least  (steels  containing 
say  less  than  0.60  per  cent  carbon),  results  are  obtained  fully  as  accurate  as  those  of 
the  colorimetric  method  and,  on  the  whole,  more  reliable,  since  the  possibility  of 
serious  errors  is  practically  eliminated.  By  the  micro-test,  for  instance,  a  steel  with 
0.25  per  cent  carbon  might  be  reported  as  containing  0.20  or  0.30  per  cent  of  that 
element,  but  it  could  hardly  be  reported  as  containing  0.15  or  0.35  per  cent.  With 
chemical  methods  on  the  contrary,  even  with  the  combustion  method,  such  errors  are 
possible,  and  occasionally  occur,  through  mechanical  loss,  faulty  manipulations,  im- 
pure reagents,  mistakes  in  weighing  or  figuring,  etc.  Chemical  analysis  calls  for  the 
complete  destruction  of  the  anatomy  of  the  metal,  destroying  at  the  same  time  evi- 
dences of  serious  error;  micrographic  analysis,  on  the  contrary,  is  based  upon  the 
anatomy  itself,  and  therefore  very  serious  errors  are  quite  impossible. 

In  order  to  yield  results  at  all  satisfactory,  however,  care  should  be  taken  that  all 
samples  be  first  annealed,  that  is  reheated  to  900  or  1000  deg.  C.  and  cooled  slowly, 
so  that  the  normal  amounts  of  pearlite  may  be  formed.  To  attempt  to  apply  the 
micro-test  to  forged  samples  for  instance  is  certain  to  lead  to  failure.  Nor  can  the 
test  be  applied  to  hypo-eutectoid  steel  containing  but  a  slight  amount  of  free  ferrite, 
for  instance  to  steel  with  from  0.60  to  0.80  per  cent  carbon,  because  of  the  difficulty 
of  estimating  accurately  the  area  occupied  by  so  small  a  proportion  of  the  constituent 
in  excess,  and  therefore  by  the  pearlite  itself.  In  the  case  of  hyper-eutectoid  steel, 
the  differences  between  the  contents  of  free  cementite  in  steels  of  materially  different 
carbon  contents  is  so  small  as  to  resist  accurate  determination.  For  instance,  steels 
respectively  with  1.10  and  1.40  per  cent  carbon  will  contain  95.3  and  90.2  per  cent 
pearlite,  a  difference  of  less  than  5  per  cent  in  their  contents  of  pearlite,  a  quantity 
too  small  to  be  estimated  with  satisfactory  accuracy  under  the  microscope. 

To  sum  up,  the  micro-test  for  the  determination  of  carbon  in  steel,  if  it  is  to  replace 
chemical  determinations,  should  be  applied  only  to  steels  containing  less  than  some 
0.60  per  cent  carbon  which  have  been  annealed  as  above  stated. 

The  author  has  found  the  following  method  to  yield  in  some  instances  satisfactory 
results:  the  sample  after  annealing  and  quick  polishing  and  etching  (a  few  small 
polishing  scratches  will  not  matter)  is  placed  under  the  microscope,  using  a  16-mm. 
objective  and  a  5X  eye-piece,  and  its  image  thrown  on  the  screen  of  the  camera.  In 
place  of  the  ordinary  camera  screen,  however,  another  screen  is  substituted  of  ground 
glass,  ruled  into  81  squares  (9X9),  so  that  every  square  covered  by  pearlite  evidently 
means  very  nearly  0.01  per  cent  of  carbon  in  the  steel  (exactly  0.01  per  cent  carbon 
if  we  assume  pearlite  to  contain  0.81  per  cent  carbon).  It  is  then  sufficient  to  esti- 
mate the  number  of  squares  occupied  by  pearlite  to  arrive  at  the  carbon  content  of 
the  steel.  The  result  may  be  checked  by  estimating  the  carbon  in  two  or  more  dif- 
ferent spots  and  reporting  the  average  if  the  agreement  is  sufficiently  close. 

Physical  Properties  of  the  Constituents  of  Steel.  —  It  will  now  be  timely  and 
profitable  to  inquire  into  the  physical  properties  of  the  three  constituents,  ferrite, 
cementite,  and  pearlite  of  which  steel  in  its  normal  condition  is  composed. 

It  will  be  evident  that  the  physical  properties  of  commercial  ferrite  must  resemble 
closely  those  of  wrought  iron  and  of  very  low  carbon  steel.  Ferrite,  therefore,  is  very- 
soft,  very  ductile,  and  relatively  weak,  having  a  ductility  corresponding  to  an  elonga- 
tion of  at  least  40  per  cent  and  a  tensile  strength  of  some  50,000  pounds  per 
square  inch.  It  is  magnetic,  has  a  high  electric  conductivity,  and  is  deprived  of 


138 


CHAPTER  VIII  —  MEDIUM   HIGH  AND   HIGH   CARBON   STEEL 


hardening  power,  industrially  speaking  at  least,  since  carbonless  iron  cannot  be  ma- 
terially hardened  by  rapid  cooling  from  a  high  temperature. 

The  properties  of  pearlite  are  evidently  those  of  eutectoid  steel  in  its  normal,  i.e. 
pearlitic  condition,  from  which  we  may  infer  that  pearlite  has  a  tenacity  of  some 
125,000  pounds  per  square  inch,  an  elongation  of  some  10  per  cent,  that  it  is  hard, 
and  for  reasons  later  to  be  explained,  that  it  possesses  maximum  hardening  power. 

With  the  exception  of  its  very  great  hardness  little  is  positively  known  as  to  the 
physical  properties  of  cementite.  It  may  be  assumed,  however,  that  so  hard  and 
brittle  a  substance  must  greatly  lack  tenacity.  Its  tensile  strength  probably  does 
not  exceed  5000  pounds  per  square  inch  and  may  be  considerably  less,  while  its  duc- 
tility must  be  practically  nil.  It  possesses  no  hardening  power. 

These  properties  of  the  constituents  of  steel  in  its  normal  condition  are  tabulated 
below: 


CONSTITUENTS 

TENSILE  STRENGTH 
LB8.  PER  8Q.  IN. 

ELONGATION 

%   IN  2   IN. 

HARDNEBS 

HARDENING  POWEH 

Ferrite 

50,000  * 

40  * 

Soft 

None 

Pearlite 

125,000  * 

10  * 

Hard 

Maximum 

Cementite 

5000  (?) 

0 

Very  hard 

None 

Tenacity  of  Steel  vs.  Its  Structural  Composition.  —  Knowing  the  physical  prop- 
erties of  the  three  constituents  of  steel,  it  should  be  possible  to  foretell  with  some 
degree  of  accuracy  the  physical  properties  of  any  steel  of  known  structural  com- 
position, on  the  reasonable  assumption  that  these  constituents  impart  to  the  steel 
their  own  physical  properties  in  a  degree  proportional  to  the  amounts  in  which  they 
are  present.  The  properties  of  steel  made  up  for  instance  of  50  per  cent  ferrite  and 
50  per  cent  pearlite  should  be  the  means  of  the  properties  of  ferrite  and  of  pearlite. 
Let  us  assume  such  reasoning  to  be  correct  and  let  us  apply  it  to  the  tensile  strength 
first  of  hypo-eutectoid  steel  and  then  of  hyper-eutectoid  steel. 

The  tensile  strength  (T)  of  any  hypo-eutectoid  steel  will  be  expressed  by  the  fol- 
lowing formula  in  terms  of  its  structural  composition,  that  is  in  terms  of  the  per- 
centages of  ferrite  (F)  and  pearlite  (P)  which  it  contains : 

50,000  F  +  125,000  P 


T  = 


100 


in  which  50,000  represents  the  tensile  strength  of  ferrite  and  125,000  the  strength  of 
pearlite. 

Or  simplifying: 

T  =  500  F  +  1250  P 

or  again  in  terms  of  pearlite  alone,  since  F  =  100  —  P 

T  =  500  (100  -  P)  +  1250  P 
or  T  =  50,000  +  750  P 

or  finally  in  terms  of  carbon,  since  P  =  120  C 

T  =  50,000  +  90,000  C 


CHAPTER  VIII  —  MEDIUM   HIGH   AND   HIGH   CARBON   STEEL  139 

On  applying  this  simple  formula  to  steels  containing  respectively  0.10,  0.25,  and 
0.50  per  cent  carbon  we  find  for  these  metals  tensile  strengths  respectively  of  59,000, 
72,500,  and  95,000  pounds  per  square  inch.  These  values  agree  closely  with  our 
knowledge  of  the  average  tenacity  of  such  steels  when  in  a  pearlitic  condition,  and 
prove  the  value  of  the  formula  derived  from  the  considerations  outlined  above  as  to 
the  relation  existing  between  the  physical  properties  of  steel  and  its  structural  com- 
position. It  should  be  borne  in  mind  that  in  working  out  this  formula  it  has  been 
assumed  that  pearlite  contains  0.834  per  cent  carbon. 

The  values  obtained  for  various  hypo-eutectoid  steels  should  be  accurate  only  for 
steel  in  what  has  been  termed  in  these  chapters  its  normal  condition,  that  is  steel 
which  has  been  worked,  reheated  to  a  high  temperature,  and  slowly  cooled.  It  should 
be  noted,  however,  as  later  explained,  that  steel  worked  and  finished  at  a  fairly  high 
temperature  is  practically  in  this  so-called  normal  condition,  so  that  the  formula 
may  be  used,  and  fair  results  expected,  to  calculate  the  tensile  strength  of  such  hot 
worked  steel.  If  the  steel  be  worked  until  its  temperature  is  quite  low  and,  especially, 
if  it  be  cold  worked,  it  is  well  known  that  its  tensile  strength  is  generally  increased. 
Neither  can  the  formula  be  used,  of  course,  in  the  case  of  hardened  steel  or  of  steel 
castings.  It  may,  however,  be  applied  to  steel  castings  which  have  been  properly 
annealed,  when  the  tensile  strength  may  be  brought  up  to  the  level  of  steel  forgings 
finished  fairly  hot  as  explained  in  another  chapter. 

Again  the  formula  is  of  value  only  in  case  of  commercial  steels  containing  the 
usual  proportions  of  impurities  especially  of  manganese.  It  applies  only  to  steels  in 
which  the  percentage  of  manganese  varies  roughly  with  the  carbon  content  from 
some  0.20  to  0.80  per  cent.  The  presence  of  a  larger  proportion  of  manganese  would 
increase  the  tenacity  materially. 

Passing  to  the  tensile  strength  of  hyper-eutectoid  steel,  our  ignorance  as  to  the 
tenacity  of  cementite  does  not  permit  the  writing  of  a  formula  with  the  same  degree 
of  confidence.  Let  us  assume,  tentatively,  however,  that  cementite  has  a  tensile 
strength  of  5000  pounds  per  square  inch  and  then  proceed  as  we  did  in  the  case  of 
hypo-eutectoid  steel. 

The  tensile  strength  of  any  hyper-eutectoid  steel  may  be  expressed  by  the  follow- 
ing formula  in  terms  of  the  percentages  of  pearlite  (P)  and  cementite  (Cm)  which  it 

C°ntainS:  =  125,000  P  +  5000  Cm 

100 
or  simplifying:  T  =  1250  P  +  50  Cm 

or  in  terms  of  pearlite  only,  since  Cm  =  100  —  P, 

T  =  1250  P  +  50  (100  -  P) 
T  =  5000  +  1200  P 

or  since,  as  previously  shown,  P  =  800  —  120  C 

? 


T  -  5000+  1200 

or  simplifying  :  ^  995,000  -  144,000  C 

7 
or  approximately  T  =  142,000  -  20,600  C. 


140  CHAPTER  VIII  — MEDIUM   HIGH  AND  HIGH  CARBON  STEEL 

Applying  this  formula  to  steels  containing  respectively  1.25  and  1.50  per  cent 
carbon,  we  find  for  their  respective  strength  116,250  and  111,100  per  square  inch, 
which  are  fair  values  for  the  average  tenacity  of  pearlitic  steels  of  those  degrees  of 
carburization.1 

Steel  of  Maximum  Strength.  —  From  the  preceding  considerations  it  seems  evi- 
dent that  eutectoid  steel  must  possess  maximum  tensile  strength  since  the  influence 
of  the  presence  of  ever  so  small  an  amount  of  free  ferrite  in  hypo-eutectoid  steel  or 
of  free  cementite  in  hyper-eutectoid  steel  must  necessarily  be  a  weakening  one,  be- 
cause of  the  relative  weakness  of  free  ferrite  and  free  cementite  as  compared  to  the 
strength  of  pearlite.  By  most  writers,  on  the  other  hand,  steel  of  maximum  tenacity 
is  often  stated  to  contain  in  the  vicinity  of  1  per  cent  carbon,  that  is  to  be  slightly 
hyper-eutectoid. 

It  is  not  clear,  however,  that  the  results  upon  which  the  statement  is  based  were 
obtained  in  testing  steel  in  its  pearlitic  condition.  On  the  contrary  it  seems  probable 
that  a  large  number  of  the  steels  tested  were  in  a  sorbitic  rather  than  in  a  pearlitic 
condition  because  of  relatively  quick  cooling  through  the  critical  range  as  explained 
in  a  subsequent  chapter.  And  while  it  appears  that  pearlitic  steel  must  have  its  maxi- 
mum tenacity  when  composed  entirely  of  pearlite,  it  may  well  be  that  when  in  a  sor- 
bitic condition  maximum  strength  corresponds  to  a  higher  degree  of  carburization, 
i.e.  1  per  cent,  because  sorbite  may  contain  and  indeed  often  does  contain  more  carbon 
than  pearlite.  Indeed  the  cases  on  record  show  that  when  the  steels  were  made  pearl- 
itic through  very  slow  cooling  maxium  tenacity  corresponds  closely  to  the  eutectoid 
composition.  Arnold,  for  instance,  tested  a  series  of  very  pure  carbon  steel  and  after 
slow  cooling  in  the  furnace  from  1000  deg.  C.  he  found  a  very  sharp  maximum  in  the 
tenacity  corresponding  to  0.89  per  cent  carbon.  On  cooling  these  same  steels  in  air, 
on  the  contrary,  and  therefore  making  them  sorbitic,  maximum  tenacity  corresponded 
to  1.20  per  cent  carbon.  Harbord  likewise  ascertained  the  tenacity  of  very  pure 
steels  and  found  after  slow  cooling  (in  the  furnace)  from  900  deg.  C  that  the  maxi- 
mum tenacity  corresponded  to  0.947  per  cent  carbon. 

1  Empirical  formulas  have  often  been  suggested  to  express  the  relation  between  the  tenacity  of 
steel  and  its  carbon  content.  Deshayes  proposed  for  unannealed  steel 

T  =  30.09  +  18.05  C  +  36.11  C2 
Thurston  (minimum  values)  for  unannealed  steel 

T =42.32  +  49.37  C 
and  for  annealed  steel 

T= 35.27  +  42.32  C 
Bauschinger  for  Bessemer  steel 

T  =  43.64  (1  +C2) 
Weyrauch  (minimum  values) 

T  =  44.17  (1  +  C) 
Salom  (average  values) 

T  =  31.74  +  70.53  C 

The  above  formulas  express  the  tenacity  in  kilograms  per  square  millimeter.     Campbell,  for  acid 
open  hearth  steel,  gives 

T  =  40,000  +  1000  C  +  1000  P  +  xMn  +  R 

and  for  basic  open  hearth  steel 

T  =  41,500  +  770  C  +  1000  P  +  yMn  +  R 

in  which  x  and  y  are  values  given  in  a  table  and  dependent  upon  the  percentage  of  manganese  and 
of  carbon  present.    R  is  a.  variable  to  allow  for  heat  treatment. 


CHAPTER   VIII  —  MEDIUM   HIGH   AND   HIGH   CARBON   STEEL 


141 


Ductility  of  Steel  vs.  Its  Structural  Composition.  —  From  the  known  ductility, 
as  expressed  by  its  elongation  under  tension,  of  ferrite  and  the  known  elongation  of 
pearlite,  respectively  40  and  10  per  cent  in  two  inches,  it  should  be  possible  to  work 
out  a  formula  expressing  the  ductility  of  any  hypo-eutectoid  steel  in  the  annealed 
(pearlitic)  condition.  In  terms  of  ferrite  and  pearlite  the  ductility  should  be 

40  F  +  10  P 


D  = 


100 


or  simplifying : 

D  =  .4F  +  .IP 

or  in  terms  of  pearlite  alone  since  F  =  100  —  P 

D  =  .4  (100  -  P)  +  .1  P  =  40  -  .3  P 

and  since  P  =  120  C,  the  ductility  in  terms  of  carbon  will  be 

D  =  40  -  36  C 

Pearlitic  steels  for  instance  containing  0.25  and  0.50  per  cent  carbon  should  have 
elongations  respectively  of  31  and  22  per  cent.1 


a 

eo    -K 


24 

sea 


30 
CO 


Fig.  155.  —  Diagram  showing  the  relation  between  the  tenacity  and  ductility  of  annealed  (pearlitic) 

steels  and  the  carbon  content. 


1  It  is  interesting  to  compare  this  formula  with  some  others  that  have  been  proposed.    Howe 
gives  for  the  elongation  in  8  inches  of  steel  under  0.50  per  cent  carbon 

D  =  33  -60  (C2  +0.1) 
and  for  steel  between  0.50  and  1.00  per  cent  carbon: 

D  =  12  -11.9-v/C  -0.5 
Deshayes  for  the  elongation  in  S  inches,  gives 

D  =  42  -56C 
and  for  the  elongation  in  4  inches 

D  =  35  -30C 

These  formulas  give  lower  values  for  the  elongation  of  steel  than  the  author's  formula,  but  all  indi- 
cations point  to  the  fact  that  they  refer  to  steel  in  a  rather  sorbitic  condition  and,  therefore  more 
tenacious  and  less  ductile,  whereas  the  formula  here  suggested  is  for  truly  pearlitic  steel  only. 


142  CHAPTER  VIII  — MEDIUM   HIGH   AND  HIGH  CARBON  STEEL 

Diagram  Showing  the  Relation  between  the  Tenacity  and  Ductility  of  Steel  and 
Its  Carbon  Content.  —  By  plotting  the  formulas  suggested  in  this  chapter  to  express 
the  relation  between  the  carbon  content  of  steel  and  its  tenacity  and  ductility  the 
curves  of  Figure  155  are  obtained.  To  the  tenacity  and  ductility  curves  a  third 
curve  has  been  added  showing  the  variation  of  the  amount  of  pearlite  with  the 
carbon  content. 


CHAPTER  IX 

IMPURITIES   IN   STEEL 

Metallic  Impurities.  —  Commercial  grades  of  steel  always  contain,  besides  carbon, 
varying  amounts  of  silicon,  phosphorus,  sulphur,  and  manganese,  often  an  appreciable 
proportion  of  copper  and  traces  at  least  of  many  other  metals  and  metalloids.  These 
may  be  called  the  metallic  impurities.  Sulphur,  however,  is  sometimes  classified  as 
a  non-metallic  impurity  as  later  explained. 

Non-Metallic  or  Oxidized  Impurities.  —  Non-metallic  or  oxidized  impurities, 
chiefly  oxides,  sulphides,  and  silicates  of  iron  and  manganese,  are  also  frequently  found 
in  steel,  principally  through  the  retention  by  the  metal  of  some  of  the  slag  produced 
during  the  refining  operation.  Hibbard  has  recently  suggested  the  name  of  "sonims" 
for  this  class  of  impurities.  They  are  also  known  as  slag  "enclosures"  or  "inclusions." 

Metallic  vs.  Non-Metallic  Impurities.  —  There  is  a  sharp  distinction  between  the 
behavior  of  metallic  and  non-metallic  impurities,  the  former,  with  the  exception  of 
sulphur,  forming  true  alloys  with  the  contaminated  metal,  the  latter  being  merely 
inclusions,  their  union  with  the  metal  being  purely  mechanical. 

Gaseous  Impurities.  —  Steel  always  contains  some  gases,  apparently  held  in  solu- 
tion and  called  "occluded"  gases,  chiefly  hydrogen,  nitrogen,  and  carbon  monoxide 
(CO). 

Impurities  vs.  Physical  Properties  of  Steel.  —  It  is  well  known  that  surprisingly 
small  proportions  of  some  of  the  metallic  impurities  just  mentioned  have  a  very 
marked  influence  upon  the  physical  properties  of  steel.  Some  0.2  per  cent  phosphorus, 
for  instance,  renders  many  grades  of  steel  so  brittle  as  to  unfit  them  for  most  com- 
mercial uses.  And  as  it  is  logical  to  suppose  that  there  exists  a  very  close  relation 
between  the  structure  of  a  metal  and  its  physical  characteristics,  we  naturally  ex- 
pect to  find  important  structural  changes  corresponding  to  marked  alterations  of 
physical  properties.  We  should  expect,  for  instance,  the  structure  of  a  high  phos- 
phorus, brittle  steel  to  be  quite  different  from  the  structure  of  a  low  phosphorus, 
tough  steel  of  otherwise  identical  composition.  In  the  present  state  of  metallography 
the  microscope  does  not  always  reveal  such  differences  of  structures  as  we  are  led  to 
look  for.  We  may  reasonably  anticipate,  however,  that,  as  the  science  progresses, 
structural  differences  will  be  detected  of  a  magnitude  fairly  in  keeping  with  the  deep 
changes  of  physical  properties  brought  about  by  slight  changes  of  chemical  composi- 
tion. Indeed  in  recent  years  material  advance  has  been  made  in  this  direction  and 
the  influence  of  the  usual  impurities  upon  the  properties  of  steel  has  been  on  the 
whole  satisfactorily  accounted  for  by  metallographic  methods  as  will  be  apparent 
from  the  description  which  follows. 

Silicon  in  Steel.  —  All  grades  of  steel  contain  a  trace  at  least  of  silicon  (Si)  and 
occasionally  as  much  as  0.5  per  cent,  and  even  more,  most  grades  containing  between 
0.05  and  0.3  per  cent. 

143 


144  CHAPTER   IX  —  IMPURITIES    IN   STEEL 

When  present  in  such  small  proportion  silicon  is  entirely  dissolved  in  the  iron 
with  which  it  forms  a  solid  solution.1  It  is  probable,  however,  that  it  is  not  held  in 
solution  by  the  iron  in  its  elementary  condition,  Si,  but  rather  as  a  silicide  of  iron, 
FeSi.2  Since  the  atomic  weight  of  iron  is  56  and  that  of  silicon  28  it  will  be  evident 
that  28  parts  by  weight  of  silicon  produces  56  +  28  or  84  parts  by  weight  of  FeSi,  or 

/  ft  -d  \ 

that  silicon  produces  exactly  3  times  its  own  weight  of  FeSi  (    -  =  3  ) .     For  instance 

\28       / 

0.1  per  cent  silicon  in  the  steel  will  give  rise  to  the  formation  of  0.3  per  cent  of  FeSi 
and  this  small  amount  of  iron  silicide  will  be  held  in  solid  solution  by  the  iron.  The 
ferrite  of  commercial  steel,  therefore,  always  contains  a  small  amount  of  silicon  in 
the  form  of  an  iron  silicide,  and  let  it  be  borne  in  mind  that  this  applies  to  the  ferrite 
forming  part  of  the  pearlite  of  all  slowly  cooled  steels  as  well  as  to  the  free  ferrite  of 
hypo-eutectoid  steel. 

It  has  been  stated  in  another  chapter  that  when  an  impurity  forms  a  solid  solution 
with  the  contaminated  metal,  changes  of  crystalline  forms  are  not  generally  ob- 
served. This  is  true  in  the  present  case  for  there  is  apparently  no  structural  difference 
between  a  steel  with  some  0.3  or  0.4  per  cent  silicon  and  a  steel  nearly  free  from  that 
element  but  otherwise  of  identical  composition.  The  presence  of  silicon  in  steel  can- 
not as  yet  be  satisfactorily  detected,  even  qualitatively,  by  metallographic  methods, 
although  we  have  the  unquestionably  accurate  statement  of  Le  Chat  flier  that  silicon 
causes  ferrite  to  etch  more  slowly. 

In  view  of  the  similarity  of  structure  between  steel  containing  much  silicon  (i.e. 
several  tenths  of  1  per  cent)  and  steel  practically  free  from  it,  we  should  expect  that 
the  presence  of  a  small  amount  of  silicon  cannot  affect  materially  the  properties  of 
steel,  and  this  we  know  to  be  the  case. 

Phosphorus  in  Steel.  —  Steel  of  satisfactory  quality  contains  from  a  trace  to  0.1 
per  cent  of  phosphorus  (P) .  As  in  the  case  of  silicon  this  small  amount  of  phosphorus 
is  held  in  solid  solution  by  the  iron,  not,  however,  in  the  elementary  state,  P,  but  as 
the  phosphide  of  iron  Fe3P.  The  atomic  weight  of  iron  being  56,  that  of  phosphorus 
31,  and  the  phosphide  containing  three  atoms  of  iron  for  each  atom  of  phosphorus,  it 
will  be  obvious  that  31  parts  by  weight  of  phosphorus  will  form  3  X  56  +  31  or  199 
parts  of  the  phosphide  Fe3P,  or  roughly,  1  part  by  weight  of  phosphorus  will  give 
rise  to  the  formation  of  6  parts  of  phosphide.  For  instance,  the  presence  in  steel  of 
0.05  per  cent  phosphorus  results  in  the  formation  of  0.3  per  cent  of  Fe3P  held  in  solid 
solution  by  the  ferrite,  this  being  true  of  the  ferrite  included  in  the  pearlite  of  all 
slowly  cooled  steel  as  well  as  of  the  free  ferrite  of  hypo-eutectoid  steel. 

While  phosphorus  in  common  with  other  metallic  impurities  forming  solid  solu- 
tions does  not  alter  the  crystalline  form  of  steel,  it  is  believed  by  some  to  have  a 
marked  tendency  to  enlarge  the  grains  of  the  metal,  which  tendency  would  account 
for  the  well-known  brittleness  imparted  to  steel  by  phosphorus  when  present  in  ex- 

1  Some  writers  believe,  apparently  on  good  ground,  that  in  oast  iron  at  least,  a  metal  which  gen- 
erally contains  a  considerable  amount  of  silicon,  a  portion  of  that  element  crystallizes  with  the 
romentite  decreasing  its  stability  as  later  explained.  In  steel,  however,  the  proportion  of  silicon 
present  is  small  and  if  any  of  it  dissolves  in  the  cementite  it  must  be  in  so  small  a  quantity  as  to  be 
negligible. 

1  Some  writers  mention  FeSi2  as  the  formula  of  the  iron  silicide  present  in  steel  but  this  con- 
tention is  not  well  supported  by  experimental  evidences. 


CHAPTER   IX  — IMPURITIES   IN   STEEL 


145 


cess  of  0.1  per  cent.  The  brittleness  caused  by  a  large  grain  will  be  considered  further 
in  another  chapter. 

Except  for  this  possible  enlargement  of  the  grains,  microscopical  examination  does 
not  reveal  the  presence  of  the  usually  small  percentages  of  phosphorus  occurring  in 
steel,  unless  it  be  segregated,  although  it  is  said  by  some  writers  that  phosphorus  as 
well  as  manganese  causes  ferrite  to  etch  darker.  The  existence  of  portions  richer  in 
phosphorus  than  others  may  be  detected  by  means  of  Stead's  reagent  as  described  in 
Chapter  II. 

Sulphur  in  Steel.  —  Steel  of  satisfactory  commercial  quality  Thay  contain  from  a 
mere  trace  to  some  0.1  per  cent  sulphur,  generally  between  0.01  and  0.05  per  cent. 
It  is  universally  known  that  manganese  and  sulphur  have  very  great  reciprocal 
affinity  so  that  when  brought  together  at  a  high  temperature  they  combine  chem- 
ically with  each  other  to  form  the  sulphide  of  manganese,  MnS.  This  is  what  happens 


Fig.  156.  —  Manganese  sulphide  in  steel  cast-       Fig.    157.  —  Steel.      Forged.      Hypo-eutectoid. 
ings.     Magnified  640  diameters.     (Boylston.)  Manganese  sulphide  in  ferrite  areas.      Magni- 

fied 300  diameters.     (Levy.) 

in  steel  which  always  contains  manganese  as  well  as  sulphur.  From  the  atomic 
weight  of  manganese,  55,  and  that  of  sulphur,  32,  it  will  be  seen  that  32  parts  by 
weight  of  sulphur  produces  87  parts  of  MnS,  or  approximately  2J^  parts  of  sulphide 
for  each  part  of  sulphur.  Steel  with  0.05  per  cent  sulphur,  for  instance,  will  contain 
about  0.125  per  cent  of  MnS,  provided,  of  course,  there  is  enough  manganese  present 
to  satisfy  the  sulphur  which  must  necessarily  be  so  in  properly  made  steel. 

The  existence  of  the  sulphide  of  manganese,  MnS,  in  steel  has  been  conclusively 
proven.  In  steel  castings  it  occurs  as  rounded  areas  the  color  of  which  is  generally 
described  as  pale  or  dove  gray  or  slate  color.  In  forgings  it  occurs  in  elongated 
particles,  bands,  or  strings  of  the  same  tint,  running  parallel  to  the  direction  of  the 
forging  or  rolling  (Figs.  156  and  157). 

According  to  Le  Chatelier  MnS  has  a  melting-point  superior  even  to  that  of  iron, 
solidifying,  therefore,  first,  and  the  bulk  of  it  rising  to  the  top  of  the  bath  or  ingot, 
the  manganese  is  in  this  way  helpful  in  removing  sulphur  from  steel.  Some  writers, 


146  CHAPTER   IX  —  IMPURITIES   IX   STEEL 

however,  question  this  higher  melting-point  of  MnS.  Levy  reports  that  the  melting- 
point  of  pure  MnS  is  probably  not  far  from  1400  cleg.  C.  and,  therefore,  below  the 
melting-point  of  hypo-eutectoid  steel  at  least,  while  the  presence  of  some  FeS  would 
lower  materially  its  melting-point.1  It  appears  probable  that  the  solidification  of  tin- 
sulphide  globules  must  follow  and  not  precede  that  of  the  iron. 

This  view  seems  to  be  supported  by  the  location  of  the  sulphide  particles  at  the 
boundaries  of  the  pearlite  grains  of  eutectoid  steel,  in  the  free  ferrite  of  hypo-eutec- 
toid steel  or  in  the  free  cementite  of  hyper-eutectoid  steel.  What  MnS  is  retained  by 
the  solid  steel,  since  it  occurs  as  shown  in  the  shape  of  small  individual  grains  or 
elongated  particles,  can  only  injure  the  metal  through  breaking  up  its  continuity 
and,  in  view  of  the  very  small  amount  of  sulphur  and,  therefore,  of  MnS,  present  in 
steel  of  good  quality,  it  is  evident  that  this  breaking  up  and  its  action  upon  the  prop- 
erties, must  be  very  slight.  This  is  in  agreement  with  the  known  fact  that  a  small 
amount  of  sulphur  in  steel  containing  also  the  proper  amount  of  manganese  has  no 
appreciably  injurious  effect. 

Seeing  that  steel  seldom  contains  much  more  than  some  0.05  per  cent  sulphur, 
hence  more  than  0.125  per  cent  MnS,  it  is  not  to  be  expected  that  this  compound  will 
always  be  detected  in  polished  and  etched  steel  sections.  Indeed  whenever  detected 
it  points  to  a  segregation  of  the  sulphide  together  with  other  impurities  (ghost  lines) 
as  described  later. 

In  case  sulphur  occurs  in  excess  over  the  amount  needed  to  form  the  sulphide 
MnS  with  the  manganese  present  in  the  steel,  the  excess  sulphur,  that  is  the  sulphur 
left  over  after  satisfying  the  manganese,  combines  with  some  of  the  iron,  forming  the 
iron  sulphide  FeS.  It  should  be  noted  at  once,  however,  that  it  requires  less  than 
2  parts  by  weight  of  manganese  (atomic  weight  55)  to  combine  with  1  part  of  sulphur 
(atomic  weight  32).  In  other  words  if  the  steel  contains  twice  as  much  manganese 
as  it  does  sulphur,  this  should  theoretically  be  enough  to  convert  the  whole  of  the 
sulphur  into  the  sulphide  MnS.  As  it  is  very  seldom  indeed  that  steel  does  not  con- 
tain a  much  larger  proportion  of  manganese  than  that  compared  to  its  sulphur  con- 
tent, the  occurrence  of  free  FeS  in  steel  should  be  very  rare.  It  is  not  to  be  expected 
in  metal  of  good  quality,  its  presence  pointing  to  a  very  abnormal  composition, 
namely,  high  sulphur  content  and  very  low  percentage  of  manganese.  According  to 
Levy,  however,  MnS  and  FeS  are  readily  soluble  in  each  other  in  the  solid  state, 
MnS  being  capable  of  holding  as  much  as  50  per  cent  of  FeS  in  solid  solution.  Accord- 
ing to  this  writer  MnS  is  seldom  free  from  FeS  even  when  the  steel  contains  consid- 
erable manganese,  the  mass  action  exerted  by  the  presence  of  so  large  a  proportion  of 
iron  preventing  the  manganese  from  taking  hold  of  the  totality  of  the  sulphur  in  spite 
of  its  greater  affinity  for  it.  MnS  nearly  free  from  FeS  has  a  clear  dove  gray  color  free 
from  yellowish  tints,  while  its  color  becomes  more  yellowish  as  the  proportion  of  FeS 
increases.  Levy  notes  also  that  in  high  carbon  steel  the  MnS  areas  are  generally 
colored  darker  than  in  low  carbon  steel,  indicating  greater  freedom  from  FeS,  ap- 
parently owing  to  the  fact  that  in  high  carbon  steel  the  mass-  action  exerted  by  iron 
is  not  so  great  since  it  contains  less  iron. 

The  sulphide  FeS  exhibits  a  marked  tendency  to  form  continuous  envelopes  or 
membranes  surrounding  each  grain  of  pearlite  (Fig.  158),  and  probably  consisting  of  a 

1  Rohl  reports  that  the  freezing-point  of  pure  MnS  is  1620  cleg.  C.  but  that  the  compound 
FejMnjSs  which,  in  his  opinion,  is  gnu-rally  the  composition  of  the  sulphide  inclusions  freezes  at 
1365  deg.  C. 


CHAPTER   IX  —  IMPURITIES   IX   STEEL  147 

eutectic  alloy  of  iron  and  iron  sulphide  (the  composition  of  the  eutectic  is  apparently: 
FeS  85  per  cent,  Fe  15  per  cent).  These  membranes  being  weak  and  brittle  impart 
weakness  and  brittleness  to  the  steel.  The  well-known  red-shortness  caused  by  sul- 
phur in  the  absence  of  a  sufficient  amount  of  manganese  (to  form  MnS)  is  probably 
due  to  the  low  melting-point  (950  deg.  C.  according  to  some  writers)  of  this  iron- 
iron  sulphide  eutectic.  At  a  high  temperature  the  melting  of  this  eutectic  destroys 
the  cohesion  between  the  grains  of  the  metal  resulting  in  cracks  being  developed  dur- 
ing the  process  of  forging  or  rolling,  and  in  extreme  cases  in  the  metal  actually  break- 
ing into  several  pieces.  The  presence  of  a  large  amount  of  FeS  in  some  Bessemer 
steel  at  the  end  of  the  blow,  before  the  addition  of  manganese,  is  undoubtedly  largely 
responsible  for  the  marked  red-shortness  of  the  metal  at  this  stage  of  the  operation. 
Under  the  microscope  FeS  appears  yellow  or  pale  brown. 


Fig.  158.  —  Red-short  steel.     Magnified  300  diameters.     Sulphur 
0.54  per  cent.     Unetched.     Network  of  FeS.     (Ziegler.) 

The  following  is  quoted  from  Stead:  "The  researches  of  Rohl  have'  demonstrated  beyond 
doubt  that  ferrous  and  manganese  sulphides  crystallize  together  in  the  proportions  of  60  per  cent 
FeS  and  40  per  cent  MnS  =  Fe3Mn2S5  and  in  mixtures  containing  increasing  quantities  of  manga- 
nese sulphide  up  to  100  per  cent,  forming  homogeneous  isomorphous  compounds  of  FesMnoSs  all  of 
which  under  the  microscope  have  the  same  appearance.  Levy  and  Law,  however,  state  that  the 
homogeneous  substance  which  contains  the  most  FeS  is  lighter  in  color  than  that  containing  less. 

When  the  FeS  exceeds  60  per  cent  in  the  mixture  a  eutectic  consisting  of  7  per  cent  MnS  and 
93  per  cent  FeS  appears,  having  a  freezing  point  of  1181  deg.  C. 

When  iron  in  excess  is  present  in  addition  to  an  excess  of  FeS  over  the  mixture  FesMnzSs,  a  ter- 
nary eutectic  is  formed  freezing  at  980  deg.  Rohl  found  the  freezing  point  of  pure  MnS  to  be  1620 
deg.,  or  about  120  deg.  higher  than  that  of  pure  iron,  and  the  freezing  point  of  the  compound 
Fe.,Mn,S5,  1365  deg. 

Judging  from  these  most  valuable  researches,  it  may  be  accepted  beyond  any  doubt  that  when 
iron  sulphide,  and  what  appears  to  be  manganese  sulphide,  are  found  associated  together  in  the  same 
sulphide  inclusions  what  looks  like  MnS  is  not  that  compound  but  is  Fe3Mn2S5." 

It  has  been  contended  by  some  that  a  portion  of  the  sulphur  present  in  steel  and 
cast  iron  is  dissolved  in  the  cementite  and  that  its  presence  increases  the  stability  of 
that  constituent.  It  seems  probable,  however,  in  view  of  the  very  small  amount 
of  sulphur  generally  present  in  steel  and  of  the  relatively  large  amount  of  manganese 


148  CHAPTER  IX  —  IMPURITIES   IN  STEEL 

that  this  possible  contamination  of  cementite  by  sulphur,  if  it  occurs  at  all,  may  be 
neglected.  This  has  been  done  in  the  present  chapter,  it  having  been  assumed  that 
the  totality  of  the  sulphur  is  present  as  MnS  or  as  MnS  and  FeS  in  the  form  of 
mechanical  inclusions. 

Sulphur  Printing.  —  As  explained  in  Chapter  II  the  presence  of  sulphur  in  steel 
and  iron,  especially  when  segregated,  may  sometimes  be  detected  by  the  taking  of 
so-called  "sulphur  prints." 

Manganese  in  Steel.  —  It  has  been  seen  that  manganese  combines  readily  with 
sulphur  and  that  the  resulting  manganese  sulphide,  MnS,  either  alone  or  combined 
with  some  FeS,  can  be  detected  in  polished  steel  sections  as  a  pale  or  dove  gray  con- 
stituent assuming  the  shape  of  rounded  areas  in  castings  and  of  bands  or  threads  in 
forgings.  Manganese  silicate  is  also  occasionally  found  in  steel  as  later  explained  and 
may  sometimes  be  mistaken  for  MnS.  Satisfactory  tests  for  the  distinction  of  these 
two  constituents  will  be  described. 

When  manganese  occurs  in  excess  over  the  amount  required  to  form  MnS  with 
the  totality  of  the  sulphur  present,  as  is  almost  universally  the  case,  the  manganese 
in  excess  combines  with  some  of  the  carbon  to  form  the  carbide  of  manganese,  Mn3C, 
and  this  carbide  is  found  associated  with  the  iron  carbide,  Fe3C,  in  cementite.  The 
cementite  of  commercial  steel,  therefore,  is  seldom  a  pure  iron  carbide,  containing 
on  the  contrary  varying  amounts  of  Mn3C.  Since  iron  and  manganese  have  practi- 
cally the  same  atomic  weight,  however  (55  and  56  respectively),  it  remains  practically 
true  that  carbon  forms  15  times  its  own  weight  of  cementite,  even  when  the  latter 
contains  a  large  proportion  of  Mn3C. 

There  is  no  metallographic  test  by  which  cementite  free  from  manganese  can  be 
distinguished  from  cementite  rich  in  Mn3C. 

Some  authors  mention  the  possible  presence  of  the  manganese  silicide,  MnSi,  in 
steel,  while  solid  solution  between  manganese  and  iron  is  frequently  referred  to. 
While  manganese  and  iron  (ferrite)  undoubtedly  form  solid  solutions,  it  does  not 
seem  likely  that  these  are  produced  when  manganese  is  present  in  small  proportion, 
say  not  over  1  per  cent.  In  that  case  it  seems  more  probable  that  manganese  is  found 
in  the  two  forms  described  above,  (1)  as  a  manganese  sulphide  MnS,  containing  prac- 
tically the  totality  of  the  sulphur  in  steel  of  good  quality,  that  is,  containing  not  over 
0.05  per  cent  sulphur  and  not  less  than  0.25  per  cent  manganese  and  (2)  as  the  man- 
ganese carbide  Mn3C,  associated  with  Fe3C  in  cementite. 

Chemical  vs.  Structural  Composition.  —  Knowing  the  probable  chemical  forms  of 
the  five  metallic  impurities  always  present  in  steel,  carbon,  silicon,  phosphorus,  sul- 
phur, and  manganese,  as  well  as  their  structural  associations,  it  will  be  interesting 
and  profitable  to  consider  accordingly  the  proximate  chemical  composition  as  well  as 
the  ultimate  and  proximate  structural  compositions  of  a  steel  of  known  ultimate 
chemical  composition.  Let  us  assume  a  steel  of  the  following  ultimate  chemical 
composition : 


c 

0.50  per  cent 

Mn 

0.80    "       " 

S 

0.05    "       " 

P 

0.04    "       " 

Si 

0.10    "       " 

Fe  (by  diff.)  98.51 
100.00 


CHAPTER   IX  —  IMPURITIES   IX   STEEL  149 

Bearing  in  mind  the  atomic  weights  of  these  elements  (Fe,  56;  C,  12;  Mn,  55;  S,  32; 
P,  31;  Si,  28)  and  the  formulas  of  the  chemical  compounds  formed  (MnS,  FeSi,  Fe3P, 
Mn3C,  Fe3C),  it  will  be  readily  seen  that: 

(1)  0.05  per  cent  S  will  give  rise  to  the  formation  of  0.13  per  cent  MnS. 

(2)  0.13  per  cent  MnS  contains  about  0.08  per  cent  Mn. 

(3)  This  leaves  0.80  —  0.08  =  0.72  per  cent  manganese  in  excess  to  combine  with  C. 

(4)  0.72  per  cent  Mn  will  form  0.77  per  cent  Mn3C. 

(5)  0.77  per  cent  Mn3C  contains  about  0.05  per  cent  carbon.. 

(6)  This  leaves  0.50  —  0.05  =  0.45  carbon  to  combine  with  iron. 

(7)  0.45  per  cent  carbon  results  in  the  formation  of  6.75  per  cent  of  Fe3C. 

(8)  0.04  per  cent  of  P  corresponds  to  about  0.25  per  cent  of  Fe3P. 

(9)  0.10  per  cent  Si  gives  0.30  per  cent  FeSi. 

The  proximate  chemical  composition  of  the  steel  considered  will  be 


Fe3C 

6.75  per  cent 

Mn3C 

0.77    "       " 

Fe3P 

0.25    "      " 

FeSi 

0.30    "      " 

MnS 

0.13    "       " 

Fe  (by  diff.) 

91.80    "       " 

100.00 

As  to  the  ultimate  structural  composition  of  the  steel,  we  know  that  the  cemen- 
tite  contains  the  Fe3C  and  the  Mn3C  hence  we  have  6.75  +  0.77  =  7.52  per  cent 
cementite.  In  pure  steel  the  percentage  of  cementite  would  have  been  0.50  X  15  = 
7.50  per  cent.  The  slight  difference  between  the  two  numbers  is  due  to  the  presence 
of  manganese  in  the  commercial  steel,  and  to  a  slight  difference  between  the  atomic 
weights  of  manganese  and  that  of  iron  (55  compared  to  56),  a  difference  so  slight 
that  for  all  practical  purposes  we  may  assume  that  in  commercial  steels  as  well  as  in 
pure  steel  the  percentage  of  carbon  multiplied  by  15  gives  the  amount  of  cementite 
formed.  The  total  ferrite  present  in  this  steel  contains  all  the  free  iron,  as  well  as  the 
small  proportions  of  Fe3P  and  FeSi  present,  hence  this  steel  contains  91.80  +  0.25  + 
0.30  =  92.35  total  ferrite.  In  pure  steel  the  proportion  of  total  ferrite  would  have 
been  100  —  7.50  or  92.50  per  cent.  The  difference  between  the  two  values  is  evi- 
dently due  to  the  presence  of  a  trifle  greater  amount  of  cementite,  and  to  the  presence 
of  0.13  per  cent  MnS. 

The  ultimate  structural  composition  of  the  steel  under  consideration  is,  there- 
Total  ferrite  92.35 
Cementite        7.52 
MnS  0.13 

100.00 

Finally  its  proximate  structural  composition  will  be,  since  the  pearlite  of  hypo- 
eutectoid  steel  contains  8  times  the  weight  of  total  cementite  (assuming  the  euter- 
toid  carbon  point  to  be  0.834  per  cent) : 

Pearlite  7.52  X  8  =  60.16 

Free  ferrite  (by  diff.)  39.71 

MnS  .13 

100.00 


150 


CHAPTER   IX  — IMPURITIES    IN.  STEEL 


Ignoring  the  presence  of  impurities  the  quick  method  described  in  Chapter  VIII 
would  have  given  pearlite  60  per  cent,  ferrite  40  per  cent,  i.e.  values  which  may  be 
considered  identical  for  any  practical  purposes.  It  follows  from  this  that  in  calcu- 
lating the  structural  composition  of  any  carbon  steel  of  ordinary  commercial  quality 
the  presence  of  the  impurities  need  not  be  considered;  the  steel  may  be  treated  as  if 
it  was  made  exclusively  of  iron  and  carbon. 

The  relation  between  chemical  and  structural  compositions,  both  ultimate  and 
proximate,  is  further  shown  in  the  following  table. 


CHEMICAL   COMPOSITION 

STRUCTURAL  TOM  POSITION 

ULTIMATE 

PROXIMATE 

ULTIMATE                                                   PROXIMATE 

% 

Fe(by  cliff.)  98.51 
Si                0.10 
P                0.04 
C                O.rtO 
Mn                0.80 
S                 0.05 

% 

Fe(bydiff.)91.80] 
FeSi  0.30  [ 
Fe3P  0.25  1 
Fe8C  6.75  \ 
MnsC  0.77  j 
MnS  0.13 

%                                                                  % 

(  Free  Ferrite                    39.71  Free 
Total  Ferrite  92.35  |                                                       Ferrite 
(  Pearlite  Ferrite  52.64  1  00.16%  Pear- 

Cementite        7.52    J                    lite 

MnS                 0.13    0.13 

100.00 

100.00 

100.00                                           100.00 

Non-Metallic  or  Oxidized  Impurities.  —  As  already  mentioned  steel  generally 
contains  varying  amounts  of  non-metallic,  oxidized  impurities  frequently  called  slag 
"enclosures,"  or  "inclusions,"  while  Hibbard  proposed  for  them  the  name  of  "sonim," 
in  which  "so"  stands  for  solid,  "n"  for  non-metallic  and  "im"  for  impurities.  They 
consist  chiefly  of  iron  and  manganese  oxides  and  silicate  although  the  sulphides  of 
iron  and  manganese  are  generally  considered  also  as  slag  enclosures.  These  impur- 
ities are  derived  mainly  (1)  from  the  retention  by  the  metal  of  minute  particles  of 
the  slag  formed  during  the  process  of  manufacture,  (2)  from  small  pieces  of  refractory 
materials  detached  from  the  linings  of  furnaces  and  ladles,  and  (3)  from  the  reaction- 
products  resulting  from  the  introduction  of  recarburizers  or  other  additions. 

According  to  Hibbard  the  formation  of  slag  enclosures  is  due  almost  entirely  to 
the  "washing"  action  of  the  additions,  principally  of  manganese,  in  combining  with 
the  dissolved  oxides,  sulphides,  and  silicates  which  are  present  in  the  steel  at  the  end 
of  the  melting  process  before  the  manganese  is  added  since  enclosures  rich  in  manga- 
nese must  necessarily  have  formed  after  the  addition  of  that  constituent.  Stead  on 
the  other  hand  expresses  the  belief  that  the  silicate  enclosures  at  all  events  are  due  to 
oxidation  of  manganese  and  silicon  occurring  during  the  passage  of  the  molten  steel 
through  the  air  in  passing  from  the  ladle  into  the  molds.  In  the  case  of  iron  oxide 
it  seems  probable  that  minute  particles  of  it  remain  in  suspension  in  the  molten 
steel  forming  as  many  minute  inclusions  after  solidification,  while  another  portion, 
dissolved  in  the  liquid  metal,  is  in  part  precipitated  during  solidification. 

In  Rosenhain's  opinion  it  is  not  proven  that  the  sulphides  and  silicates  are  not 
soluble  in  molten  steel. 

After  solidification  the  association  between  the  slag  enclosures  and  the  steel  re- 
mains a  purely  mechanical  one;  they  commonly  occur  as  rounded  or  elongated  par- 
ticles embedded  in  the  metal. 


CHAPTER   IX  —  IMPURITIES   IX   STEEL  151 

MnS  can  generally  be  readily  distinguished  from  other  inclusions  because  of  its 
characteristic  dove  gray  color,  its  globular  shape  in  castings  (Fig.  156)  and,  owing 
to  its  plasticity,  the  ease  with  which  it  is  elongated  in  forgings  (Fig.  157).  It  seems 
reasonable  to  assume  that  the  paler  its  color  the  greater  its  purity,  and  that  a  yellowish 
tint  points  to  the  presence  of  some  FeS.  Pure  FeS,  a  very  rare  constituent  of  steel, 
has  a  decidedly  yellow  color  and  is  more  brittle.  MnS  generally  occurs  in  the  free  fer- 
rite  of  hypo-eutectoid  steel,  between  the  pearlite  grains  of  eutectoid  steel  or  in  the 
free  cementite  of  hyper-eutectoid  steel. 

Silicates  are  decidedly  darker  than  MnS  (Figs.  159  and  160)  which  affords  a  means 
of  distinguishing  between  them  even  when  they  are  associated  in  the  same  particles 
as  it  sometimes  happens  (Fig.  159).  Silicates  because  of  their  relative  brittleness  are 


. 

Fig.  159.  —  Manganese  sulphide  (light  constituent)  and  manganese  sili- 
cate in  steel.    Magnified  1000  diameters.     (Law.) 

frequently  broken  and  torn  by  the  forging  operation  (Fig.  160).  Sometimes  the  man- 
ganese sulphide  forms  dendrites  embedded  in  a  silicate  matrix. 

Stead  recommends  the  placing  of  a  drop  of  sulphuric  acid  on  the  polished  speci- 
men, when  H2S  gas  will  be  evolved  where  MnS  is  present,  particles  of  silicates  of 
manganese,  on  the  contrary,  evolving  no  gas.  The  dissolving  of  MnS  also  leaves  pits. 
Stead  also  advises  heat  tinting  as  the  best  means  of  distinguishing  between  the  sul- 
phide and  the  silicate,  the  heating  to  be  continued  until  the  specimen  has  assumed 
a  light  brown  coloration,  when  the  MnS  remaining  bright  can  be  sharply  differen- 
tiated from  the  silicate. 

No  very  satisfactory  metallographic  tests  have  so  far  been  found  to  distinguish 
between  oxides  and  silicates  or  between  iron  silicate  and  manganese  silicate.  Mat- 
weieff's  attempts  in  that  direction  have  been  described  in  Chapter  VI. 

They  are  summed  up  by  Rosenhain  as  follows:  "According  to  their  behavior  under 
these  reagents  (hydrogen  and  superheated  steam),  the  enclosures  may  be  divided  into 
three  groups  or  classes. 


152 


CHAPTER   IX  —  IMPURITIES   IN   STEEL 


"A.  Stable  bodies  which  are  not  acted  upon  either  by  hydrogen  at  300°  C,  by 
superheated  steam,  or  by  weak  organic  acids.  These  are  the  silicates  of  iron  and 
manganese. 

"B.  Bodies  which  are  reduced  to  the  metallic  state  by  the  action  of  hydrogen  at 
300°  C  and  are  acted  upon  by  steam  but  are  unaffected  by  weak  organic  acids.  These 


Fig.  160.  —  Manganese  sulphide  (light  constituent)  and  iron  silicate  in 
mild  steel.     Unetched.     Magnified  1000  diameters.     (Law.) 


Fig.  161.  —  Ghost  lines  in  low  carbon  steel.     Magnified 
95  diameters.     (Boylston.) 


are  the  oxides  of  iron  and  manganese.  Oxide  of  manganese  is  not  reduced  by  hy- 
drogen when  by  itself,  but  in  the  presence  of  iron  oxide  reduction  of  both  metals  takes 
place.  On  re-polishing  a  sample  after  heating  in  hydrogen,  the  regions  previously 
occupied  by  oxides  appear  as  bright  metal,  like  the  ferrite  of  the  surrounding  iron  or 
steel,  but  the  presence  of  manganese  may  be  detected  by  etching  with  very  dilute 


CHAPTER   IX  —  IMPURITIES   IN   STEEL 


153 


alcoholic  solution  of  ferric  chloride.  Iron  free  from  manganese  is  only  very  slightly 
colored,  but  if  manganese  is  present,  the  reagent  produces  rapid  coloration. 

"C.  Not  affected  by  hydrogen  or  steam,  but  attacked  by  weak  organic  acids 
(such  as  tartaric  acid).  These  are  the  sulphides  of  iron  and  manganese;  the  former 
is  rapidly  colored  by  this  acid  while  sulphide  of  manganese  is  only  slowly  attacked. 
The  behavior  of  the  sulphides  which  are  mixtures  or  solid  solutions  of  the  iron  and 
manganese  sulphides  is  not  considered  by  Matweieff,  but  in  the  light  of  the  work 
of  Levy  these  mixed  sulphides  require  particular  study  from  this^  point  of  view." 

Rosenhain  rightly  adds  that  the  use  of  gaseous  reagents  at  moderately  high 
temperatures  is  a  somewhat  cumbrous  manipulation,  particularly  because,  if  general 
tarnishing  of  the  polished  specimens  is  to  be  avoided,  the  hydrogen  must  be  specially 


Fig.  162.  —  Ghost  lines  in  low  carbon  steel.     Magnified  2000  diameters. 
Manganese  sulphide  and  pearlite  particles.     (Law.) 


purified.  It  is  to  be  hoped,  therefore,  that  future  research  may  yet  find  simpler  means 
of  identifying  these  substances. 

At  the  end  of  the  refining  operation  by  which  steel  is  produced,  especially  towards 
the  latter  part  of  the  Bessemer  blow,  a  considerable  amount  of  iron  oxide  is  formed 
and  in  spite  of  the  steps  taken  for  removing  it  from  the  bath  (addition  of  manganese, 
etc.)  some  of  it,  occasionally  quite  a  little,  is  retained  by  the  metal,  when  it  is  a  source 
of  red-shortness  besides  having  other  detrimental  effects.  This  iron  oxide  generally 
occurs  as  small  dark  points  visible  in  the  polished  section  before  etching. 

Segregation  of  Impurities.  Ghosts.  —  The  very  small  proportions  of  impurities 
generally  found  in  steel  of  good  quality  have  little,  if  any,  injurious  effect  upon  its 
most  important  and  useful  physical  properties,  so  long  as  they  remain  uniformly 
distributed  throughout  the  metallic  mass,  i.e.  so  long  as  the  steel  is  chemically  homo- 
geneous. These  impurities,  on  the  contrary,  may  become  extremely  injurious  when 
they  show  a  tendency  to  "segregate,"  i.e.  to  collect  in  certain  portion  or  portions  of 
steel  castings  and  forgings,  when  the  segregated  portions  may  contain  so  large  an 


154 


CHAPTER   IX  —  IMPURITIES    IN   STEEL 


amount  of  impurities  as  to  have  their  useful  properties  utterly  destroyed.     Segre- 
gated metal  is  generally  brittle,  weak,  and  hard. 

Under  the  microscope  a  metal  suffering  from  this  segregation  of  impurities  gen- 
erally is  found  to  contain  bands  of  varying  widths  and  lengths,  technically  known  as 


Fig.  163.  —  Ghost  lines  in  low  carbon  steel.     Magnified  10  diameters.     (Law.) 


-^5r!^w^?<  -%^%.:?  ?  5  •  *% 

"3«  -  ^rO?  J  "/   *.'*«?.    it^V  °K  "'^•- 

&tLf^^-l-%fS^^- 
;  ^%f4^£^%4?^ 

^r      ^g?  .- . V'rsL.  • .-"'-  '~  - '  " 


Fig.  164.  —  Ghost  lines  in  low  carbon  steel.     Magnified  200  diameters.     (Law.) 

"ghosts"  or  "ghost  lines,"  in  which  the  presence  of  abnormally  large  proportions  of 
MnS  and  phosphorus  can  generally  be  detected  by  the  ordinary  metallographic  tests. 
Photomicrographs  of  ghost  lines  are  shown  in  Figures  161  to  167.  Ghost  lines  etch 
more  rapidly  than  the  surrounding  metal  therefore  appearing  darker  after  etching  even 


CHAPTER  IX  —  IMPURITIES   IN   STEEL 


155 


to  the  naked  eye.    These  lines  can  generally  be  detected  before  etching  because  of  the 
manganese  sulphide  which  they  contain. 

Stead  describes  "ghost  lines"  as  lines  of  ferrite  in  which  are  embedded  lenticullar 
particles  of  drawn  out  sulphide  inclusions  and  he  adds  that  when  manganese  sulphide 
is  found  segregated  it  may  be  taken  for  granted  that  phosphorus  also  is  segregated 
in  the  same  regions.  He  believes  that  the  white  ferrite  lines  sometimes  observed  in 
forgings  arc  due  to  phosphorus  segregation,  the  presence  of  that  element  having  ex- 
pelled the  carbon  on  slow  cooling.  He  writes  that  the  higher  the.  phosphorus  in  the 
steel  the  thicker  and  more  pronounced  are  these  white  carbonless  lines  in  forged 


A  B 

Fig.  105.  —  Forged  steel  containing  about  0.3  per  cent  carbon  and 
0.3  per  cent  phosphorus. 

A.  Polished  and  etched  in  the  ordinary  way. 

B.  Heat-tinted  until  the  phospho-ferrite  bands  are  oxidized  to  a 

brown  tint.     (Stead.) 


steels.  In  Figure  165  Stead  shows  the  effect  of  heat  tinting  specimens  exhibiting 
these  ferrite  bands. 

Stead's  cupric  reagent  (Chapter  II)  is  also  very  effective  in  revealing  the  existence 
of  ferrite  bands  rich  in  phosphorus.  The  lighter  portions  contain  the  most  phos- 
phorus, copper  being  precipitated  on  the  portions  freer  from  that  element. 

Rosenhain  and  Haughton  likewise  bring  out  clearly  the  banded  structure  of  cer- 
tain steels  by  the  electro-chemical  deposition  of  copper  from  a  solution  of  ferric 
chloride  and  hydrochloric  acid  containing  a  small  amount  of  copper  chloride.  This 
reagent,  Rosenhain  writes,  develops  the  banded  structure  of  steel  containing  phos- 
phorus in  a  striking  manner,  as  illustrated  in  Figure  166.  Figure  167  shows  another 
typical  instance  of  a  banded  structure. 

Rosenhain  accepting  Ziegler's  view  that  the  particles  of  slag  enclosures  act  as  so 
many  nuclei  for  the  crystallization  of  ferrite  in  hypo-eutectoid  steel  and  of  cementite 


156 


CHAPTER   IX  —  IMPURITIES   IN   STEEL 


in  hyper-eutectoid  steel  argues  that  the  "banded"  structure  of  some  forged  steels 
may  be  accounted  for  through  the  crystallizing  of  ferrite  around  strings  of  enclosures 
formed  by  the  rolling  operation.  He  writes:  "Again  we  commonly  find  in  commercial 
steel  a  banded  arrangement  of  ferrite  and  pearlite,  such  as  that  shown  in  Figure  167 
and  although  this  is  obviously  traceable  to  the  rolling  process  which  the  steel  has 
undergone,  the  banded  structure  is  extremely  persistent  in  spite  of  repeated  anneal- 
ing. If  we  consider  that  the  enclosures  present  in  the  steel  have  been  rolled  out  into 
long  lines,  and  that  each  time  the  steel  cools  down  through  Ar,  the  ferrite  tends  to 
deposit  upon  these  lines  of  enclosures,  the  persistent  recurrence  of  these  bands  is 
explained.  Heat  treatment  could,  in  that  case,  only  destroy  these  bands  if  time 
enough  were  allowed  for  the  gradual  migration  of  the  enclosures  at  a  high  tempera- 


Fig.  166.  —  Banded  structure  in  steel 
containing  phosphorus.  (Rosen- 
hain.) 


V  %^«*^  ~»  F  ^?^v  *c  <^— ^ 

j>    f _^r       •>  •••••-       -   T»f  %  *-  -  — 

•  :^S^»  ^>V  v  fr$  f^ir\ 

^*       •-+  S.J^*  <  ^or    *  s>* 

^Jfir^*~&  CTA  jc---^"-.  **  "^ 


:*«? 


^-.•^~^.  «**^%/ 
^r  — r— 


Fig.  167.  —  Banded  structure  in  steel.  Magnified 
100  diameters.  (L.  T.  Holt,  Correspondence 
Course  student.) 


ture  — •  and  very  prolonged  annealing  does  break  up  the  banded  structure.  The  oc- 
currence and  persistence  of  the  comparatively  wide  carbonless  bands,  studded  with 
enclosures,  which  are  sometimes  termed  'ghosts'  may  be  accounted  for  in  a  similar 
way  by  the  original  rolling  out  into  a  long  band  of  an  austenite  boundary  containing 
a  comparatively  large  mass  of  enclosure." 

Rosenhain  would  account  as  follows  for  the  persistence  of  ferrite  bands  rich  in 
phosphorus:  "In  the  case  of  steel,  phosphorus  occurs  typically  in  this  way,  being 
present  in  solid  solution  in  the  ferrite  of  the  ingot,  but  in  the  form  of  solid-solution 
cores,  so  that  the  phosphorus  content  of  each  crystal  increases  from  its  center  to  its 
periphery.  When  rolled  out,  these  crystal  cores  assume  the  form  of  elongated  masses, 
and  although  the  ferrite  itself  undergoes  complete  re-crystallization,  possibly  re- 
peatedly, there  is  nothing  to  cause  the  phosphorus  to  migrate  except  the  process  of 
diffusion,  which  is  particularly  slow  in  that  case.  The  result  is  that  in  the  finished 
material  the  phosphorus-rich  ferrite  still  remains  in  long  bands  or  streaks,  and  these 


CHAPTER   IX  —  IMPURITIES   IN   STEEL  157 

bands  pass  indifferently  through  numbers  of  individual  crystals  —  indeed,  an  indi- 
vidual crystal  may  lie  partly  within  and  partly  outside  one  of  these  bands  —  the 
growing  ferrite  crystal  has  simply  used  the  material  it  found  at  hand,  whether  rich 
in  phosphorus  or  not." 

Gaseous  Impurities.  —  It  has  not  been  possible  so  far  to  detect  the  presence  of 
occluded  gases  in  steel  by  means  of  metallographic  methods.  While  the  problem 
seems  a  very  difficult  one  to  solve,  the  statement  that  it  can  never  be  solved  would 
not  be  justified  for  the  discovery  of  some  metallographic  treatment  by  which  a  metal 
rich  in  certain  gases  may  be  distinguished  from  a  similar  metal  free  from  them  is  well 
within  the  limits  of  reasonable  expectation. 


CHAPTER  X 

THE  THERMAL  CRITICAL   POINTS   OF  STEEL 

THEIR  OCCURRENCE 

The  structure  of  steel  described  in  the  preceding  chapter,  i.e.  its  normal  structure, 
is  greatly  affected  by  the  treatment  or  treatments,  both  mechanical  and  thermal,  to 
which  the  metal  may  be  subjected  during  the  process  of  manufacture  of  finished 
objects.  It  is  to  the  close  relation  existing  between  the  treatment  and  the  structure 
on  the  one  hand,  and  between  the  structure  and  the  physical  properties  of  the  metal 
on  the  other,  that  metallography  owes  its  industrial  importance.  It  is  essential, 
therefore,  that  the  student  should  have  a  clear  understanding  of  these  relations.  As 
a  preparation  to  this  important  study,  however,  it  will  be  necessary  to  describe  a 
phenomenon  of  the  greatest  moment  in  the  treatment  of  steel,  namely,  the  occurrence 
of  spontaneous  absorptions  or  evolutions  of  heat  during  the  heating  or  cooling  of 
the  metal.  These  are  generally  termed  the  "thermal"  critical  points  or  simply 
"critical  points,"  also  "retardations,"  "transformation"  points,  and  "critical  tem- 
peratures." 

Point  of  Recalescence.  —  If  a  piece  of  steel  containing  some  0.60  per  cent  carbon 
be  heated  to  a  high  temperature,  say  to  1000  dog.  C.,  and  allowed  to  cool  slowly 
from  that  temperature,  and  if  its  rate  of  cooling  be  carefully  ascertained,  conveniently 
by  means  of  a  Le  Chatelier  pyrometer,  it  is  found  that  the  cooling  proceeds  at  first  at 
a  nearly  uniformly  retarded  rate.  If,  for  instance,  it  requires  10  seconds  for  the  metal 
to  cool  through  the  first  five  degrees  (from  1000  to  995  deg.),  and  12  seconds  to  cool 
through  the  next  five  degrees  (995  to  990  deg.),  it  will  require  some  14  seconds  for 
the  next  five  degrees,  16  seconds  for  the  following  five,  and  so  on,  the  cooling  through 
each  range  of  five  degrees  being  a  little  slower  than  the  preceding  cooling  of  five 
degrees.  All  cooling  bodies,  whatever  their  nature,  generally  follow  this  law.  The 
plotting  of  time  and  temperature  as  coordinates  yields  smooth  curves,  sometimes 
approaching  straight  lines  (see  curve  B,  Fig.  177). 

In  the  case  of  the  steel  we  are  now  considering,  when  a  certain  temperature  is 
reached,  in  the  majority  of  cases  some  675  to  725  deg.  C.,  a  most  interesting  and 
significant  phenomenon  takes  place ;  the  cooling  of  the  metal  is  momentarily  arrested, 
the  pyrometer,  for  a  certain  length  of  time,  failing  to  record  any  further  fall  of  tem- 
perature. Indeed,  when  the  circumstances  are  favorable,  the  temperature  of  the 
cooling  mass  actually  rises;  the  metal  becomes  visibly  hotter;  it  "recalesces,"  hence 
the  name  of  "recalescence"  given  to  this  thermal  critical  point.  If  the  experiment 
be  conducted  in  a  dark  room,  this  recalescence  or  spontaneous  glow  of  the  steel  is 
plainly  visible.  After  a  while  the  metal  resumes  its  normal  rate  of  cooling  which  is 
then  continued  down  to  atmospheric  temperature. 

It  is  evident  that  at  this  critical  point  the  surrounding  atmosphere  does  not  cease 

1.58 


CHAPTER  X  — THE   THERMAL   CRITICAL   POINTS   OF  STEEL  159 

to  abstract  heat  from  the  piece  of  steel  and,  since  its  temperature  nevertheless  re- 
mains stationary  or  even  rises,  it  must  be  that  heat  is  here  spontaneously  generated 
within  the  metal  in  amount  sufficient  to  make  up,  or  more  than  make  up,  for  the  heat 
lost  by  radiation  and  conductivity. 

In  heating,  as  might  be  expected,  the  reverse  phenomenon  takes  place:  an  absorp- 
tion of  heat  causing  a  retardation  in  the  rise  of  the  temperature,  or  even  a  momentary 
stop,  the  pyrometer  failing  for  a  few  moments  to  record  any  further  increase  of 
temperature  or  recording  only  an  abnormally  low  increase,  although  heat  continues 
to  be  applied  to  the  steel  at  the  same  speed.  Actual  lowering  of  the  temperature  of 
the  steel  is  not  generally  observed  at  this  critical  point  on  heating,  i.e.  the  steel  does 
not  grow  perceptibly  colder. 

Notation.  —  Osmond,  who  was  the  first  to  determine  accurately  the  position  and 
magnitude  of  the  point  of  recalescence  and  who  is  the  discoverer  of  the  upper  critical 
points  soon  to  be  described,  adopted  Tschernoff  s  previous  notations,  and  designated 
the  critical  points  by  the  letter  A.1  To  distinguish  critical  points  on  cooling  from 
those  occurring  on  heating  the  former  are  called  Ar  (from  the  French  refraidissement, 
meaning  cooling)  and  the  latter  Ac  (from  the  French  chauffage,  heating).  To  dis- 
tinguish further  between  the  point  of  recalescence  and  its  reversal  on  heating  on  the 
one  hand,  and  critical  points  occurring  at  higher  temperatures  on  the  other,  the  nota- 
tions Ari  and  Aci  are  used  for  the  recalescence  point  and  its  reversal,  and  Ar2,  Ac2, 
Ar3,  Ac3,  for  the  two  upper  reversible  critical  points  soon  to  be  described.  The  nota- 
tions Ai,  A2,  A3,  are  frequently  used  when  the  points  and  their  reversals  are  consid- 
ered collectively.  By  the  notation  AI,  for  instance,  is  meant  the  point  of  recalescence 
Ari  and  its  reversal  Aci.  These  notations  will  be  used  in  these  chapters. 

Brinell,  in  his  important  work  on  the  heat  treatment  of  steel,  used  the  letter  V  for 
the  point  of  recalescence  and  W  for  its  reversal  on  heating.  These  symbols,  however, 
are  now  very  seldom  used. 

The  expression  "point  of  recalescence"  is  frequently  used  indifferently  for  the 
point  on  cooling,  where  heat  is  evolved  causing  a  recalescence  of  the  metal,  and  for 
the  reverse  phenomenon  on  heating,  at  Aci,  where,  of  course  instead  of  a  recalescence 
taking  place,  an  absorption  of  heat  occurs  causing  the  metal  to  lose  heat.  It  is  ob- 
vious that  the  term  "recalescence"  should  not  be  applied  to  the  point  Aci.  The  point 
of  recalescence  is  also  called  sometimes  " recalescent "  point  and,  seldom,  "Gore's  phe- 
nomenon" (see  Historical  Sketch  at  end  of  chapter).  The  point  Aci  has  been  called 
point  of  "  decalescence "  by  some  writers  and  one  of  them  at  least  refers  to  it  as  the 
"calescence"  point. 

Critical  Range.  —  Transformation  Range.  —  When  the  various  critical  points  oc- 
curring in  steel  are  considered  collectively  the  range  of  temperature  they  cover  is 
frequently  called  the  "critical  range,"  or,  more  seldom,  but  very  appropriately,  the 
"transformation  range."  It  will  soon  be  shown  that  the  critical  range  may  include 
one,  two,  or  three  critical  points.  The  meaning  of  the  expressions  "critical  range  on 
heating"  and  "critical  range  on  cooling"  is  obvious. 

Positions  of  Ari  and  Aci.  —  The  critical  points  Ari  and  Aci  do  not  occur  at  ex- 
actly the  same  temperature,  Aci  being  generally  situated  some  20  to  40  deg.  higher 
than  Ari.  When  the  point  Ari,  for  instance,  is  found  at  690  deg.  C.,  the  point  Aci 
will  generally  occur  somewhere  between  710  and  730  degrees. 

1  The  point  A  of  Tschevnoff  indicated  the  temperature  at  which  steel  suddenly  acquires  harden- 
ing properties  on  heating  or  loses  them  on  cooling. 


100  CHAPTER   X  — THE   THERMAL   CRITICAL   POINTS   OF   STEEL 

Stead  submitted  samples  of  very  pure  steel  containing  0.9  per  cent  carbon  to  16 
well-known  investigators  with  the  request  that  they  ascertain  the  Aci  and  Ari  points. 
The  reported  results  indicated  for  Aci  temperatures  varying  between  719  and  740 
deg.,  and  for  Ari  temperatures  between  097  and  721.  This  apparently  great  dis- 
crepancy may  be  accounted  for  in  part  at  least  by  the  influence  of  the  rate  of  heating 
and  cooling  on  the  positions  of  Aci  and  Ari  respectively,  as  later  noted,  and  by  the 
influence  of  the  temperature  from  which  cooling  starts  on  the  position  of  Ari.  Unless 
standard  conditions  are  maintained  in  regard  to  rate  of  heating  and  cooling,  tem- 
perature from  which  cooling  begins  and  length  of  time  at  that  temperature,  very 
close  agreement  between  various  investigators  is  not  to  be  expected. 

It  is  also  essential  that  the  temperatures  recorded  should  correspond  to  exactly 
the  same  stage  of  the  critical  point,  namely  its  beginning  or  its  apex.  Howe  argues 
that  the  beginning  of  Ari  and  Aci  should  be  taken  as  indicating  the  position  of  these 
points  on  the  ground  that  the  beginning  of  the  transformation  is  less  affected  by  lag. 
Rosenhain,  on  the  contrary,  and  his  views  are  shared  by  the  author,  contends  that 
the  peaks  should  be  read  since  they  represent  the  temperatures  at  which  the  bulk  of 
the  specimen  undergoes  transformation. 

In  commercially  pure  carbon  steels  Ari  almost  always  occurs  between  690  and 
720  deg.  and  Aci  some  20  to  40  deg.  higher.  The  fact  that  the  critical  point  on  cooling 
lags  behind  the  point  on  heating  and  vice  versa,  is  evidently  a  case  of  hysteresis  so 
often  observed  in  physical  phenomena  and  which  implies  a  resistance  of  certain 
bodies  to  undergo  a  certain  transformation,  when  theoretically  the  transformation 
is  due,  the  delayed  transformation  finally  taking  place  with  added  violence.  This 
was  vividly  depicted  by  Howe  some  twenty  years  ago  in  the  case  of  iron.  He  wrote : 
"Just  as  we  can  cool  water  below  its  freezing-point  without  completely  freezing  it, 
thereby  rapidly  increasing  the  strength  with  which  the  water  tends  to  freeze,  so  by  a 
relatively  rapid  cooling  we  can  carry  the  metal  considerably  below  Ari,  without  giv- 
ing the  Arx  change  time  to  proceed  far,  strengthening  the  while  the  tendency  toward 
this  change,  which  keeps  kindling  more  and  more  till  it  bursts  into  a  blaze,  with  such 
evolution  of  heat  as  actually  to  recalesce,  to  raise  the  temperature  of  the  metal  by 
some  10  deg.,  in  spite  of  the  continued  abstraction  of  heat  by  the  continued  cooling 
of  the  furnace." 

The  slower  the  heating  and  cooling  the  nearer  will  the  two  points  approach  each 
other,  so  that  with  infinitely  slow  cooling  and  heating  they  would  undoubtedly  occur 
at  exactly  the  same  temperature.  If  there  remained  any  doubt  as  to  the  points  Aci 
and  Ari  representing  the  opposite  phases  of  the  same  phenomenon,  i.e.  of  A  being 
a  reversible  point,  it  would  suffice  to  dispel  it  to  consider  the  fact  that  in  order  to 
induce  the  retardation  Art  the  steel  must  first  be  heated  past  the  point  Aci;  and  re- 
ciprocally the  retardation  Act  cannot  take  place  unless  the  metal  has  first  been  cooled 
to  a  point  below  Ari.  To  illustrate :  the  melting  of  ice  and  the  freezing  of  water  are 
undoubtedly  the  opposite  phases  of  the  same  phenomenon,  each  one  undoes  the 
work  of  the  other,  and  in  order  to  freeze  the  water  we  must  first  melt  the  ice  and 
likewise  to  melt  the  ice  the  water  must  first  be  frozen;  one  change  cannot,  be  induced 
unless  the  opposite  one  has  last  taken  place.  Indeed  it  is  possible  through  very  slow 
and  undisturbed  cooling  to  lower  the  temperature  of  water  below  its  freezing-point 
before  it  starts  freezing,  a  clear  instance  of  hysteresis,  although  in  this  case  called 
"surfusion,"  and  when  freezing  takes  place  the  temperature  of  the  water  rises  to  its 
normal  freezing-point,  a  clear  case  of  recalescence  although  deprived  of  glow. 


CHAPTER    X  — THE   THERMAL   CRITICAL   POINTS   OF   STEEL 


161 


The  diagram  shown  in  Figure  168  illustrates  further  this  reversibility  of  the  point 
AI.  Let  two  parallel  lines  represent  the  phases  Ac  and  Ar  of  the  critical  point.  Let 
condition  A  represent  the  state  of  the  metal  stable  above  Ac  and  condition  B  the 
state  of  the  metal  stable  below  Ar.  The  gap  between  Ac  and  Ar  is  due  to  hysteresis. 
MN  is  the  temperature  at  which  both  the  Ac  point  and  the  Ar  point  would  occur  if 
there  was  no  hysteresis  as,  for  instance,  if  the  metal  could  be  heated  and  cooled  in- 
finitely slowly.  Assuming  the  metal  to  be  in  condition  B  at  a,  below  Ar,  on  heating 
it  from  a  to  b  above  Ac  on  reaching  the  Ac  point  at  x  it  passes  from_the  condition  B 
to  the  condition  A  with  absorption  of  heat  causing  a  retardation  in  the  heating;  on 
cooling  from  b  to  c,  that  is  to  a  temperature  below  Ac  but  above  Ar,  the  condition  A 
is  retained  so  that  upon  heating  from  c  to  d  no  transformation  can  take  place  at  x' 

Cond/f/on    A 


M- 


-/v 


A, 

due  fo 

/-/ysferes/s 


Cone// f /  on 


168.  —  Diagram  showing  reversible  critical  point. 


on  passing  through  the  Ac  point  and  therefore  no  critical  point  observed.  If  the 
metal  be  cooled  from  d  to  e,  however,  on  passing  through  Ar  at  y  it  changes  from 
condition  A  to  condition  B  with  evolution  of  heat,  causing  a  retardation  in  the  rate 
of  cooling;  if  it  now  be  heated  again  from  e  to/  above  Ac,  a  critical  point  will  be  ob- 
served at  x"  since  the  metal  now  in  condition  B  will  pass  to  condition  A. 

It  will  be  evident  that  between  Ar  and  Ac  the  metal  may  be  in  condition  A  or 
condition  B  depending  upon  whether  it  was  last  cooled  from  above  Ac  or  heated 
from  below  Ar. 

Howe  uses  the  notation  Aei  for  the  equilibrium  temperature  for  AI  that  is,  for  the 
temperature  at  which  the  transformation  Ar!  and  its  reversal,  Aci,  would  both  take 
place,  the  former  on  cooling  and  the  latter  on  heating,  in  the  complete  absence  of 
hysteresis.  From  a  very  careful  weighing  of  the  available  data  Howe  concludes  that 
the  position  of  this  equilibrium  temperature  is  probably  723  deg.  C. 

Speed  of  Cooling  and  Heating  vs.  Position  of  A,.  —  It  has  been  seen  that  the 


162  CHAPTER  X  — THE   THERMAL   CRITICAL  POINTS   OF   STEEL 

faster  the  cooling  the  lower  is  the  position  of  the  point  Art  and  the  faster  the  heating 
the  higher  the  point  Aci,  that  is,  the  faster  the  cooling  and  heating  the  greater  the 
gap  between  the  opposite  phases  Ari  and  Aci  of  the  reversible  point  AI. 

The  cooling  of  a  piece  of  steel  may  be  so  rapid,  as  in  quenching,  as  to  prevent 
altogether  the  retardation  Ari  from  taking  place,  because  a  low  temperature  is  so 
quickly  reached  that  the  rigidity  of  the  metal  prevents  the  transformation  of  which 
Ari  is  a  manifestation.  In  other  words  time  and  a  certain  amount  of  plasticity  are 
required  for  the  transformation  Ari  to  occur,  and  in  quenching  time  is  denied  when 
the  metal  is  sufficiently  plastic  (i.e.  at  a  red  heat),  while  when  time  is  given  (i.e.  after 
quenching)  the  metal  has  lost  its  plasticity.  It  remains  untransformed  or  but  par- 
tially transformed.  It  will  be  shown  in  another  chapter  that  this  suppression  of  the 
point  Ar!  is  probably  the  cause  of  the  hardening  of  carbon  steel  by  sudden  cooling.1 

Le  Chatelier  rightly  reminds  us  that  the  speed  of  the  transformations  occurring 
at  the  critical  points  of  steel  follows  the  general  laws  which  govern  the  speed  of  all 
chemical  phenomena.  In  other  words  that  the  speed  of  the  transformation  is  the 
greater  (1)  the  higher  the  absolute  temperature  and  (2)  the  wider  the  range  between 
the  actual  temperature  and  the  temperature  of  equilibrium,  that  is  the  temperature 
at  which  the  transformation  is  due.  Above  the  critical  temperature  both  influences 
act  in  the  same  direction  and  the  speed  of  transformation  increases  without  limit. 
Below  the  critical  temperature  these  influences  act  in  opposite  directions  necessarily 
giving  rise  to  the  existence  of  a  maximum  speed.  According  to  Le  Chatelier  this 
notion  of  variable  speeds  of  transformation  accounts  for  all  the  peculiarities  of  the 
hardening  treatment.  On  heating  it  is  hardly  possible  to  raise  the  temperature  of 
transformation  more  than  100  deg.  C.  through  very  rapid  heating,  while  during  cool- 
ing the  speed  reaches  its  maximum  at  about  600  deg.  C.,  is  very  feeble  below  200,  and 
nearly  null  at  atmospheric  temperature. 

Temperature  from  which  Cooling  begins  vs.  Position  of  Ar!.  —  It  was  first  observed 
by  Tschernoff  that  as  the  temperature  to  which  steel  is  heated  before  being  allowed 
to  cool  increases,  the  position  of  the  Ari  point  is  gradually,  although  slowly,  lowered. 
This  influence  of  what  Howe  calls  the  maximum  temperature  or  Tmax,  has  been  re- 
peatedly confirmed.  Benedicks  would  explain  it  on  the  assumption  that  at  a  higher 
temperature  a  greater  number  of  nuclei  or  centers  of  crystallization  are  destroyed 
and  that  the  fewer  nuclei  left  the  lower  the  temperature  at  which  recalescence  occurs. 
Howe  believes  that  the  phenomenon  may  be  further  explained  (1)  through  the  steep- 
ening of  the  thermal  gradient  during  the  critical  transformation  caused  by  cooling 
from  a  higher  temperature,  (2).  through  increase  of  internal  pressure  resulting  from 
the  steepening  of  the  thermal  gradient,  and  (3)  through  a  better  diffusion  of  the  pro- 
eutectoid  element  and  the  pearlite  element,  so  that  their  re-assembling  for  the  purpose 
of  undergoing  the  transformation  in  cooling  is  more  difficult. 

A  long  sojourn  at  a  high  temperature  also  appears  to  have  a  tendency  to  lower 
Ari,  while  according  to  Rosenhain  the  position  of  Aci  is  slightly  raised  if  the  previous 
cooling  has  been  very  slow  and  vice  versa. 

Chemical  Composition  vs.  Position  of  AL  —  Generally  speaking  impurities  have  a 
tendency  to  lower  the  position  of  Aci  and  Ari,  some  of  them  decidedly. 

According  to  Howe  manganese  lowers  the  Ari  point  some  24  to  50  deg.  C.  for  each 
per  cent  of  manganese,  while  Osmond  indicates  the  position  of  Ari  in  a  steel  con- 

1  It  will  be  explained  later  that  some  writers  have  doubted  the  suppression  of  the  transforma- 
tions on  rapid  cooling  and  have  suggested  another  explanation  of  the  hardening  of  steel. 


CHAPTER  X  — THE   THERMAL  CRITICAL   POINTS   OF   STEEL  163 

taining  1  per  cent  of  Mn  as  685  deg.  whereas  with  4  per  cent  of  manganese  the  same 
point  was  lowered  to  590  deg.  It  is  conceivable  that  further  increase  of  that  element 
must  lower  still  more  the  critical  point,  so  that  finally  it  may  be  lowered  below  at- 
mospheric temperature,  being  apparently  eliminated.  It  will  be  shown  in  another 
chapter  that  this  is  precisely  what  occurs  in  the  cases  of  manganese  steel  and  high 
nickel  steel,  containing  respectively  some  13  per  cent  of  manganese  or  some  25  per  cent 
of  nickel.  These  steels  exhibit  no  retardation  on  cooling  from  a  high  temperature  to 
atmospheric  temperature.  When  cooled  to  lower  temperatures,  however,  by  immers- 
ing them  in  freezing  mixtures  or,  if  need  be,  in  liquid  air,  the  retardations  may  again 
occur,  at  least  in  the  case  of  nickel  steel. 

G.  H.  Clevenger  found  that  4.5  per  cent  copper  lowers  the  Ari  point  of  steel  con- 
taining some  0.5  per  cent  carbon  from  730  to  635  deg.  C.  or  approximately  20  deg. 
for  each  per  cent  of  copper.  According  to  J.  H.  Andrews  the  absorption  of  0.25  per 
cent  of  nitrogen  by  a  steel  containing  0.6  per  cent  carbon  lowers  the  point  Ari  in  a 
marked  degree. 

In  the  case  of  commercial  steel  of  good  quality  the  proportion  of  impurities, 
with  the  possible  exception  of  manganese,  varies  within  relatively  very  narrow  limits, 
so  that  no  great  variation  should  be  expected  in  the  position  of  the  critical  point  AI. 
Neither  is  it  clear  that  the  amount  of  carbon  present  in  steel  haj.  a  marked  effect 
upon  the  position  of  the  point  AI,  although  some  writers  state  that  the  point  is  lifted 
as  the  carbon  increases.  As  previously  stated  in  pure  carbon  steel  the  point  Ari  al- 
most invariably  occurs  somewhere  between  690  and  720  deg.  C.  and  its  reversal  Ad 
20  to  40  deg.  higher.  Both  Ari  and  Aci  would  probably  occur  at  about  720  deg.  could 
the  cooling  and  heating  be  infinitely  slow. 

Upper  Critical  Points.  —  The  existence  of  upper  critical  points,  that  is,  of  thermal 
retardations  occurring  at  temperatures  higher  than  that  of  the  recalescence  point,  has 
already  been  alluded  to.  These  points  were  discovered  by  Osmond  and  their  dis- 
covery ushered  in  a  new  epoch  in  the  scientific  study  of  iron  and  steel.  To  describe 
these  points  it  is  advisable  to  consider  first  the  thermal  retardations  occurring  in 
cooling  and  heating  carbonless  iron  and  then  similar  retardations  exhibited  by  steel 
containing  increasing  amounts  of  carbon. 

Thermal  Critical  Points  in  Pure  Iron.  —  On  cooling  from  a  high  temperature,  say 
1000  deg.  C.,  a  piece  of  the  purest  iron  obtainable  and  ascertaining  its  rate  of  cooling 
as  previously  explained,  the  metal  is  found  to  cool  normally,  i.e.  at  a  uniformly  re- 
tarded rate,  until  a  temperature  of  some  900  to  880  deg.  C.  is  reached  when  a  marked 
retardation  is  observed  in  the  rate  of  cooling,  indicating  a  spontaneous  evolution  of 
heat,  in  this  case,  however,  insufficient  to  cause  an  actual  rise  of  temperature,  i.e.  a 
recalescence  of  the  metal.  The  cooling  then  resumes,  or  nearly  resumes,  a  normal 
rate  of  cooling,  until  at  about  760  deg.  C.  a  second  evolution  of  heat  takes  place 
causing  another  retardation  in  the  rate  of  cooling,  not  so  marked,  however,  nor  so 
sharply  defined  as  the  first  one.  The  metal  then  cools  normally  or  quite  so  to  atmos- 
pheric temperature.  We  have  thus  detected  two  unmistakable  spontaneous  evolu- 
tions of  heat  in  the  cooling  of  pure  iron.  The  corresponding  critical  points  are  called 
Ar3  and  Ar2,  the  latter  symbol  indicating  the  lower  point.  It  should  be  noted  that 
the  recalescence  point  which  should  occur  at  some  700  deg.  is  here  absent.  Carbon- 
less iron  has  no  point  of  recalescence. 

These  two  upper  points  like  the  point  of  recalescence  are  reversible  critical  points, 
i.e.  on  heating  the  opposite  phases  of  the  transformations  (whatever  those  trans- 


164  CHAPTER   X  — THE   THERMAL   CRITICAL   POINTS   OF   STEEL 

formations  may  be)  take  place  with  absorption  of  heat,  causing  a  retardation  in  the 
rate  of  heating  and  the  corresponding  points  being  designated  by  the  symbols  Ac3 
and  Ac2.  The  point  Ac3  occurs  at  a  temperature  some  10  to  30  deg.  higher  than  its 
reversal  Ar3,  while  Ac2  occurs  at  nearly  the  same  temperature  as  Ar2. 

Dr.  J.  K.  Burgess  and  J.  J.  Crowe  (Bulletin  American  Inst.  Mining  Engineers, 
October  1913,  p.  2537)  made  a  large  number  of  determinations  of  the  critical 
points  of  the  purest  irons  obtainable  using  great  refinement  in  manipulations, 
and  found  Ac3  to  occur  at  909  deg.  C.,  Ar3  at  898  deg.  and  Ac2  and  Ar2  both  at 
768  deg. 

Peculiarities  of  the  Point  A2.  —  The  point  A2  is  generally  less  marked  than  the 
points  A3  and  AI.  Unlike  A3  its  position  is  little  affected  by  the  carbon  content,  and 
unlike  A3  and  A!  the  point  on  heating,  Ac2,  occurs  at  nearly  the  same  temperature  as 
the  point  on  cooling,  Ar2.  To  these  peculiarities  must  be  added  another  one,  namely, 
the  fact  that  A2  appears  to  cover  a  wide  range  of  temperature.  While  its  intensity 
decreases  with  fall  of  temperature  its  lower  limit  probably  extends  to  considerably 
below  700  deg.  In  other  words  the  transformation  of  which  A2  is  a  manifestation  is 
not  completed  by  the  time  the  point  AI  is  reached.  Indeed  Osmond  mentions  550 
deg.  C.  as  the  probable  lower  limit  of  the  point  A2.  Some  explanations  of  these  pecu- 
liarities of  the  point  A2  will  soon  be  offered. 

Arnold  insists  that  with  satisfactory  apparatus  the  point  Ar2  always  shows  a 
double  peak,  the  upper  one  at  about  765  deg.,  the  lower  at  752  deg.  This  statement 
is  generally  opposed  by  the  results  of  other  investigators.  None  of  the  130  curves, 
for  instance,  taken  with  the  greatest  care  and  skill  by  Dr.  Burgess  shows  a  double 
peak  at  Ar2. 

Thermal  Critical  Points  in  Very  Low  Carbon  Steel.  —  Let  us  now  take  a  sample 
of  steel  containing  some  0.10  per  cent  carbon,  and  let  us  ascertain  its  rate  of  cooling 
from  a  high  temperature  precisely  as  before.  Three  thermal  retardations  will  be  de- 
tected, Ar3  at  about  850  deg.,  Ar2  near  760  deg.,  and  A^  (point  of  recalescence)  near 
700  deg.  Of  these  three  spontaneous  evolutions  of  heat  the  upper  one  at  Ar3  will  be 
the  most  marked,  while  at  Ar2  and  at  An  they  will  be  quite  faint,  their  satisfactory 
detection  calling  for  the  use  of  delicate  instruments  and  careful  manipulations.  On 
heating  corresponding  retardations  will  occur,  due  to  spontaneous  absorptions  of 
heat,  the  resulting  critical  points  being  designated  as  Ac3,  Ac2,  and  Aci.  Of  these 
Ac3  and  Aci  will  occur  at  temperatures  some  20  deg.  or  more  higher  than  Ar3  and  Ari, 
while  Ac2  will  occupy  nearly  the  same  position  as  Ar2  on  the  temperature  scale,  that 
is  about  760  deg. 

Thermal  Critical  Points  of  Medium  High  Carbon  Steel.  —  The  determination  of 
the  rate  of  cooling  of  a  steel  containing  some  0.45  per  cent  carbon  reveals  the  exist- 
ence of  two  critical  points,  one,  evidently  the  point  of  recalescence,  Ari,  at  the  usual 
temperature  (680  to  720  deg.)  and  one  upper  point  in  the  vicinity  of  740  deg.  Does 
the  presence  in  this  steel  of  only  one  upper  point  mean  that  one  of  the  two  upper 
points  detected  in  carbonless  iron  and  in  very  low  carbon  steel  has  disappeared, 
because  of  the  presence  of  more  carbon,  or  does  it  mean  that  the  two  upper  points 
have  now  united  into  a  single  one?  The  latter  view  is  generally  assumed  to  be  the 
correct  one  and  this  single  upper  point  of  medium  high  carbon  steel  is  designated 
accordingly  by  Ar3.2.  This  notation  clearly  implies  that  the  two  distinct  evolutions 
of  heat  which  in  carbonless  iron  and  in  very  soft  steel  occur  separately  at  Ar3  and 
Ar2  here  occur  at  one  and  the  same  temperature.  Increasing  the  carbon  content 


CHAPTER   X  — THE   THERMAL   CRITICAL   POINTS   OF   STEEL  165 

decreases  the  interval  of  temperature  between  the  two  upper  points  until,  finally,  for 
a  certain  carbon  content  the  points  meet  to  form  the  double  point  Ar3.2. 

Merging  of  A3  and  A2.  —  It  has  been  seen  that  as  the  carbon  increases  the  point 
A3  is  gradually  lowered  until  finally  it  merges  with  A2,  whose  position  is  not  greatly 
affected  by  the  presence  of  carbon,  to  form  the  point  A3.2-  It  would  be  interesting  to 
know  the  exact  proportion  of  carbon  required  to  cause  this  merging.  This,  however, 
is  difficult  to  ascertain  because  of  the  experimental  difficulty  of  separating  two  crit- 
ical points  situated  very  near  each  other  as  they  must  be  in  thejvicinity  of  the  merg- 
ing point,  and  also  because  this  merging  will  be  shifted  somewhat  by  speed  of  heating 
and  cooling  and  by  slight  changes  of  chemical  composition.  According  to  A.  Meu- 
then's  calorimetric  work  later  referred  to,  A3  and  A2  merge  for  a  carbon  content 
corresponding  to  0.32  per  cent,  while  Howe  indicates  0.438  per  cent  carbon  as  the  prob- 
able merging  point.  From  the  mass  of  experimental  evidences  which  have  been  pub- 
lished it  seems  probable  that  in  commercial  steels  the  merging  takes  place  at  about 
0.30  per  cent  carbon. 

Equilibrium  Temperatures  for  A;t  and  A3.2.  —  Howe  uses  the  notations  Ae3  for 
the  equilibrium  temperature  for  A3,  that  is  the  temperature  at  which  the  transforma- 
tion Ar3  on  cooling  and  its  reversal,  Ac3  on  heating,  would  take  place  in  the  complete 
absence  of  hysteresis.  A  close  examination  of  the  available  data  leads  to  the  con- 
clusions that  the  position  of  Ae3  is  given  by  the  formula 

T°  =  917  -  306  X  C 

in  which  C  represents  the  percentage  of  carbon  in  the  steel  and  T  the  temperature  in 
degrees  C.  for  Ae3. 

For  the  equilibrium  temperature  of  A3.2  Howe  gives  the  formula 

T°  =  820  -  105.5  X  C 

Thermal  Critical  Point  in  Eutectoid  Steel.  —  Eutectoid  steel,  that  is  steel  con- 
taining some  0.85  per  cent  of  carbon,  exhibits  but  one  critical  point,  the  point  of 
recalescence,  very  marked  at  about  700  deg.  C.  on  cooling.  Shall  it  be  inferred  from 
the  occurrence  of  this  single  point  that  in  eutectoid  steel  the  transformations  of  which 
the  upper  points  A3  and  A2  or  the  double  point  A3.2  are  manifestations  do  not  take 
place?  Or  shall  it  be  assumed  that  these  transformations  now  take  place  at  the  same 
temperature  as  the  transformation  corresponding  to  the  critical  point  AI?  In  other 
words  that  increasing  the  amount  of  carbon  has  so  depressed  the  position  of  the  two 
upper  points  as  to  cause  them  to  unite  with  the  lower  point,  forming  now  a  triple 
point  to  be  designated  as  Ar3.2.i?  This  is  the  view  generally  held.  The  critical  point 
on  heating  is  designated  by  the  notation  Ac3.2.i.  It  will  be  explained  in  another 
chapter  why  the  points  A3,  A2,  or  A3.2  cannot  exist  in  eutectoid  or  hyper-eutectoid 
steel,  when  it  will  also  be  shown  that  the  single  point  of  eutectoid  steel  is  not  in  fact 
a  merging  of  A3,  A2,  and  AI,  but  merely  the  point  AI,  the  upper  points  having  dis- 
appeared. 

Merging  of  A3.2  and  AI.  —  As  the  carbon  content  of  the  steel  increases  still  more 
after  the  merging  of  A3  and  A2  has  been  effected,  the  interval  between  the  points 
A3.2  and  AI  gradually  diminishes  until  these  two  points,  in  turn,  appear  to  merge  to 
form  the  triple  point  A3.2.i.  Theoretically  this  apparent  merging  should  occur  when 
the  steel  is  composed  entirely  of  pearlite,  that  is,  when  it  contains  in  the  vicinity  of 
0.85  per  cent  carbon,  for  reasons  that  will  later  be  made  clear.  As  a  matter  of  fact, 


166  CHAPTER  X  — THE   THERMAL   CRITICAL   POINTS   OF   STEEL 

however,  the  merging  seems  to  take  place  long  before  so  large  a  proportion  of  carbon 
is  present,  for  the  point  A3.2  is  seldom  detected  in  steel  containing  more  than  some 
0.50  or  0.60  per  cent  of  carbon;  this  is  probably  due,  as  already  explained,  to  the 
difficulty  of  separating,  experimentally,  two  critical  points  so  close  to  each  other. 

Factors  Influencing  the  Positions  of  the  Upper  Points  A3  and  A2.  —  It  has  been 
made  clear  that  as  the  carbon  increases  the  point  A3  is  gradually  lowered  until  with 
some  0.30  or  0.40  per  cent  carbon  it  merges  with  the  A2  point,  while  the  resulting 
double  point  A3.2  is  further  lowered  with  increasing  carbon  and  merges  with  the  AI 
point  at  about  0.85  per  cent  carbon.  The  position  of  the  A2  point  on  the  contrary  so 
long  as  that  point  remains  independent,  that  is  in  alloys  containing  less  than  some 
0.40  per  cent  carbon,  is  unaffected  by  variations  in  carbon  content.  Like  the  points 
of  recalescence,  the  upper  points  on  cooling  Ar3,  Ar2,  and  Ar3.2  are  probably  lowered 
(1)  by  rapid  cooling,  (2)  by  increasing  the  temperature  from  which  cooling  starts, 
(3)  by  a  long  sojourn  at  a  high  temperature,  and  (4)  by  the  presence  of  notable  pro- 
portions of  some  elements  such  as  manganese,  nickel,  silicon,  etc.,  while  the  cor- 
responding points  on  heating  Ac3,  Ac2,  and  Ac3.2  are  probably  slightly  raised  by  in- 
creased speed  of  heating. 

P.  Oberhoffer  reports  that  one  per  cent  of  manganese  lowers  the  point  A3  or  A3.a 
about  70  deg.  C.  while  Charpy  and  A.  Cornu  (Comptes  Rendus,  1913,  Vol.  CLVII, 
p.  319)  found  (1)  that  the  point  A3  of  iron  and  low  carbon  steel  vanishes  when  the 
silicon-content  reaches  1.5  per  cent  and  (2)  that  the  point  A2  remains  distinct  but 
that  each  increase  of  one  per  cent  of  silicon  lowers  its  position  by  about  11  deg.  C. 

Thermal  Critical  Points  in  Hyper-Eutectoid  Steel.  —  Carefully  conducted  obser- 
vations reveal  the  existence  of  an  upper  critical  point  in  hyper-eutectoid  steel,  at 
least  in  steel  containing  a  decided  amount  of  free  cementite  and,  of  course,  of  the 
point  of  recalescence.  It  seems  proper  to  designate  this  upper  point  by  the  symbol 
Acm  (Arcm  on  cooling,  Accm  on  heating)  for  reasons  later  to  be  given,  cm  standing 
for  cementite.  At  least  one  writer,  however,  has  designated  this  point  on  cooling 
by  the  notation  Armo,  me  standing  for  massive  cementite.  Other  writers  have  called 
it  an  A3  point,  a  notation  from  which  one  would  naturally  infer  that  this  upper  point 
of  hyper-eutectoid  steel  is  similar  to  the  upper  point  of  iron  and  of  very  low  carbon 
steel,  which  is  not  the  case. 

Purely  theoretical  considerations  lead  us  to  infer  that  the  position  of  the  point 
Acm  is  lowered  as  the  proportion  of  carbon  decreases,  finally  merging  with  the  point 
Aa.2.1  at  the  eutectoid  point.  It  would  follow  from  this  that  the  single  point  of  eu- 
tectoid  steel  is  really  a  merging  of  four  points  A3,  A2,  Ai,  and  Acm  and  that  it  should 
accordingly  be  designated  by  A3.2.icm.  It  is,  however,  the  universal  custom  to  ignore 
this  contribution  of  Acm  to  the  single  point  of  eutectoid  steel  and  to  use  for  the  latter 
the  notation  A3.2.i. 

The  amount  of  heat  evolved  at  Arcm  is  very  slight,  hence  the  difficulty  of  detect- 
ing this  point.  Carpenter  and  Keeling  ascertained  its  existence  in  steels  containing 
respectively  1.31,  1.51,  1.69,  1.85,  and  1.97  per  cent  carbon  at  the  following  corre- 
sponding temperatures:  883,  911,  985,  1030,  and  1042  deg.  C.  With  lower  carbon 
contents,  that  is  nearer  the  eutectoid  composition,  the  heat  evolved  is  so  slight  that 
the  detection  of  Arcm  as  a  separate  point  is  quite  impossible.  In  theoretical  dia- 
grams, however,  the  existence  of  this  point  is  always  indicated  in  all  hyper-eutectoid 
steels  with  a  sharp  merging  with  A3.2.i  at  the  eutectoid  point. 

The  point  Arcm  then  should  occur  in  all  hyper-eutectoid  steels  at  temperatures 


CHAPTER  X  — THE   THERMAL   CRITICAL   POINTS   OF   STEEL  167 

increasing  from  some  700  to  1050  deg.  C.  as  the  carbon  increases  from  0.85  to  2.00 
per  cent. 

Merging  of  A3.2.i  and  A,.m.  —  As  already  explained  theoretically  the  merging  of 
the  points  A3.2.i  and  Acm  should  take  place  at  the  eutectoid  composition,  that  is,  for 
steel  containing  in  the  vicinity  of  0.85  per  cent  carbon.  Experimentally,  however, 
the  point  Acm  cannot  be  detected  in  steel  containing  less  than  some  1.20  per  cent 
carbon.  Bearing  in  mind  that  hypo-eutectoid  steels  containing  more  than  0.60  per 
cent  carbon  or  thereabout  have  likewise  but  one  critical  point  so^far  as  experimental 
evidences  are  concerned,  it  will  be  seen  that  for  all  practical  purposes  we  may  con- 
sider all  grades  of  steel  containing  from  0.60  to  1.20  per  cent  carbon  as  having  but 
one  critical  point,  namely,  the  point  of  recalescence,  at  some  700  deg.  C.  on  cooling, 
although  theoretically  eutectoid  steel  only  should  have  but  one  such  point. 

Minor  Critical  Points.  —  Some  experimenters  believe  to  have  discovered  some  critical  points 
other  than  those  so  far  described.  These  points,  which  may  be  referred  to  as  minor  critical  points, 
correspond  to  very  faint  evolutions  or  absorptions  of  heat,  and  produce,  therefore,  but  very  slight 
jogs  in  the  thermal  curves.  Their  existence  is  not  fully  established  and  they  appear  to  have  but 
little  if  any  influence  upon  the  practical  side  of  our  subject.  They  should,  however,  be  mentioned  in 
these  pages  so  that  the  student  may  at  least  have  some  idea  of  their  nature  and  claims  to  recogni- 
tion. Roberts-Austen  in  1898  detected  a  slight  evolution  of  heat  between  550  and  600  on  cooling  in 
iron  and  hypo-eutectoid  steel,  and  this  point  was  again  detected  by  Carpenter  and  Keeling  in  1904. 
The  latter  observers  named  it  the  Ar0  point,  following  in  this  Roberts-Austen. 

Roberts-Austen  detected  another  evolution  of  heat  in  pure  iron  between  450  and  500  deg.  C. 
the  existence  of  which  he  ascribed  to  the  presence  of  hydrogen  resulting  in  a  separation  of  hydroxide 
of  iron  taking  place  at  this  critical  point.  Finally  the  same  observer  described  one  more  slight  evo- 
lution of  heat  in  pure  iron  at  about  270  deg.  C.  which  he  tentatively  ascribed  to  the  formation  of  an 
iron-iron  hydroxide  eutectic. 

Arnold  believes  in  the  existence  of  a  critical  point  between  A3  and  A2,  of  maximum  intensity 
when  the  steel  contains  some  50  per  cent  of  pearlite  (about  0.45  per  cent  carbon)  which  he  thinks  is 
due  to  the  formation  or  segregation  of  pearlite  and  hardenite,  a  constituent  later  to  be  described. 

Data  Showing  the  Position  of  the  Critical  Points.  —  A  very  comprehensive  set 
of  determinations  of  the  critical  points  of  iron  and  steel  was  made  by  Carpenter  and 
Keeling.  Their  results  are  tabulated  on  page  168.  The  table  includes  the  criti- 
cal points  occurring  during  the  solidification  period  of  the  various  steels  and  irons 
investigated.  These  will  be  considered  in  another  chapter.  The  position  of  the 
critical  points  as  determined  by  Burgess,  Stead,  and  others  has  already  been  indica- 
ted. There  is  substantial  agreement  between  the  results  of  these  investigators. 

Relative  Quantities  of  Heat  Evolved  or  Absorbed  at  the  Critical  Points.  —  The 
various  critical  points  that  have  been  considered  in  the  preceding  pages  do  not  indi- 
cate evolutions  or  absorptions  of  equal  quantities  of  heat;  they  are  not  of  equal  in- 
tensity. The  point  A3  is  very  marked  and  sharply  defined  in  carbonless  iron  but 
decreases  rapidly  in  intensity  as  the  carbon  increases.  The  point  A2  is  relatively 
feeble  and  not  very  sharply  denned  and  as  already  mentioned  shows  a  tendency  to 
cover  a  considerable  range  of  temperature.  Its  intensity  moreover  is  little  affected 
by  the  carbon  content  of  the  steel.  The  point  A3.2,  being  a  merging  of  A3  and  A2,  is 
more  intense  than  A2  but  less  intense  than  A3  in  carbonless  iron  owing  to  the  fact 
that  when  the  merging  takes  place  the  A3  point  has  lost  much  of  its  intensity.  The 
point  Ai  is  feeble  in.  very  low  carbon  steel  but  its  intensity  increases  rapidly  with  the 
carbon  content,  becoming  so  great  as  to  cause  the  metal  to  glow  or  recalesce  as  pre- 
viously described  and  being  maximum  for  steel  of  eutectoid  composition.  These 


168 


CHAPTER  X  — THE   THERMAL   CRITICAL   POINTS   OF   STEEL 


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CHAPTER  X  — THE   THERMAL   CRITICAL   POINTS   OF   STEEL  169 

differences  in  the  thermal  values  of  the  critical  points  will  be  explained  in  another 
chapter. 

A.  Meuthen  (Ferrum,  1912,  Vol.  X,  p.  1)  found  by  careful  calorimetric  experi- 
ments (1)  that  one  gram  of  pearlite  requires  15.9  calories  for  its  transformation  into 
austenite  (a  transformation  which  occurs  at  Aci  or  Ac3.2.i,  as  explained  in  the  follow- 
ing chapter) ;  (2)  that  the  transformation  of  one  gram  of  13  iron  into  a  iron  develops 
5.6  calories  (a  transformation  which  occurs  at  Ar2);  (3)  that  the  transformation  (at 
Ar3)  of  one  gram  of  7  iron  into  ft  iron  also  develops  5.6  calories;  arid_(4)  that  the  sep- 
aration of  one  gram  of  a  iron  (free  ferrite)  from  the  solid  solution  (austenite),  a 
separation  which  occurs  between  Ar3  and  An.  produces  14.1  calories. 

Graphical  Representation  of  the  Position  and  Magnitude  of  the  Critical  Points. 
The  position  of  the  critical  points  corresponding  to  various  percentages  of  carbon  is 
illustrated  graphically  in  Figure  169.  The  diagram  refers  to  the  critical  points  in 
cooling,  i.e.  the  Ar  points,  and  it  should  be  borne  in  mind  that  the  corresponding 
points  on  heating,  the  Ac  points,  occur  some  20  to  40  deg.  higher,  with  the  exception 
of  the  point  Ac2  which  seems  to  occupy  nearly  the  same  position  as  the  point  Ar2. 
An  attempt  has  been  made  in  this  diagram  to  indicate  the  relative  intensities  of  the 
various  points  by  shaded  areas  of  proportional  thickness  on  both  sides  of  the  lines 
indicating  their  position.  This  is  based  chiefly  on  theoretical  considerations  and  is 
in  accordance  with  the  generally  accepted  views  regarding  the  causes  of  the  critical 
points  as  explained  in  the  next  chapter.  An  examination  of  the  diagram  shows  (1) 
that  the  point  Ar3  intense  in  carbonless  iron  decreases  gradually  in  intensity  as  the 
carbon  increases,  (2)  that  the  intensity  of  Ar2  is  not  greatly  affected  by  the  carbon 
content,  (3)  that  Ar3.2  fairly  intense  at  first  becomes  rapidly  feebler  and  finally  dis- 
appears just  as  it  meets  Ari,  (4)  that  Ari  at  first  very  faint  becomes  more  marked 
with  increased  carbon,  being  maximum  for  a  carbon  content  of  some  0.85  per  cent 
(the  eutectoid  point),  (5)  that  the  point  Ar3.2.i  very  intense  at  the  eutectoid  point 
gradually  loses  some  of  its  intensity,  although  always  remaining  pronounced,  and 
(6)  that  the  point  Arcm  very  faint  near  the  eutectoid  composition  increases  in  in- 
tensity with  the  carbon  content. 

These  theoretical  inferences  are  well  supported  by  experimental  evidences  in  the 
case  of  the  magnitude  of  the  points  Ari  and  Ar3.2.i  and  quite  satisfactorily  in  regard 
to  Ar3  and  Arcm.  They  ascribed  to  the  points  Ar2  and  Ar3.2,  however,  a  magnitude 
and  a  sharpness  which  is  not  borne  out  by  experiments  as  later  explained  when  it 
will  also  be  seen  that  some  writers  doubt  the  accuracy  of  the  explanation  generally 
offered  to  account  for  the  point  A2. 

The  diagram,  therefore,  while  undoubtedly  useful,  is  probably  but  approximately 
accurate  and  likely  to  be  modified  with  increased  knowledge  of  the  facts  it  aims  to 
depict. 

Determination  of  the  Thermal  Critical  Points.  —  The  thermal  critical  points  are 
universally  determined  by  means  of  the  Le  Chatelier  thermo-electric  pyrometer. 
Indeed  it  is  the  invention  of  this  invaluable  instrument  that  made  the  detection  of 
the  upper  critical  points  possible.  Had  it  not  been  invented  we  probably  would  still 
be  in  ignorance  of  the  existence  of  the  upper  points,  while  we  would  have  but  little 
knowledge  of  the  exact  position  of  the  point  of  recalescence. 

Cooling  and  Heating  Curves.  —  The  determination  of  the  thermal  critical  points 
calls  for  the  construction  of  heating  and  cooling  curves.  In  these  curves  successive 
falls  (or  rises)  of  temperature,  say  of  10  deg.  C.,  6  —  10,  6  —  20,  6  —  30  . . .  are  plotted 


170 


CHAPTER  X  —  THE   THERMAL   CRITICAL   POINTS   OF   STEEL 


o  o 


CHAPTER  X  — THE   THERMAL   CRITICAL   POINTS   OF   STEEL 


171 


as  ordinates,  while  as  abscissae  are  plotted  (a)  the  corresponding  time  intervals  in 
seconds,  t,  t',  t",  t'"  .  .  .  elapsed  since  the  beginning  of  the  observation,  or  (b)  the 
actual  intervals  of  time  t'  —  t,  t"  -  t',  t'"  -  t"  .  .  .  required  for  each  noted  fall  of 
temperature.  In  other  words  the  coordinates  are  6  and  t  in  the  first  instance,  6  and 
dt  in  the  second.  The  curve  obtained  by  the  first  method  is  known  as  a  time-tem- 
perature curve  while  the  second  method  yields  an  inverse  rate  curve.1 

Time-temperature  curves  representing  the  heating  and  cooling  of  pure  iron  are 
shown  in  Figure  170.    While  in  these  curves  the  evolutions  or  absorptions  of  heat  cor- 


1600 
14CO 
•1300 

1300 

foe 

s, 

g  ION 
MO 

800 
700 

too 
M» 

s 

Solid!  n 

nation  PC 

int 

\ 

l 

A 

Iron 

j 

3 

A 

t 

1 

\ 

/& 

\ 

\ 

/ 

// 

006°4- 

\ 

**Cg       , 

/ 

^^^    88< 

°     ^r3 

800° 

Acs 

\ 

v  |3  Iron 

/ 

1 

fltX^ 

\ 

// 

\ 

fC  Iron 

/ 

^ 

X 

W             20              SO              40             60              60              70             80              X            100 

Minutes 

Fig.  170.  —  Time-temperature  curves.     Heating  and  cooling  of  pure  iron. 

(Goerena.) 


responding  to  the  points  A3  and  A2  can  be  detected,  they  do  not  stand  out  very  con- 
spicuously, and  it  may  well  be  feared  that  slight  thermal  retardations  might  escape 
detection  in  curves  of  this  kind  since  they  would  cause  but  very  slight  jogs  in  the 
curve.  These  considerations  led  Osmond  to  adopt  the  inverse  rate  method  for  the 
plotting  of  thermal  curves.  Curves  of  this  type  are  shown  in  Figures  171  and  172. 

1  It  is  evident  that  similar  curves  would  result  from  reversing  the  observations,  i.e.  noting  the 
successive  falls  of  temperatures  6,  6',  6"  .  .  .  corresponding  to  equal  intervals  of  time,  say  of  15 
seconds,  t  +  15,  t  +  30,  t  +  45  .  .  .  and  plotting  the  former  as  ordinates  and  the  latter  as  abscissa;. 
The  coordinates  in  this  case  would  be  dS  and  t. 


172 


CHAPTER   X  — THE   THERMAL   CRITICAL   POINTS   OF   STEEL 


Fig.  171.  —  Inverse  rate  curves.     Cooling  of  steels  containing  respectively 
0.02,  0.14,  0.45,  and  1.24  per  cent  carbon.     (Osmond.) 


CHAPTER   X  — THE   THERMAL   CRITICAL   POINTS   OF   STEEL 


173 


The  thermal  points  correspond  to  sharp  peaks  in  the  curves,  the  lengths  of  which 
are  roughly  proportional  to  the  amount  of  heat  evolved  on  cooling  or  absorbed  on 
heating.  This  method  is  quite  universally  applied  unless,  as  later  explained,  a  neutral 
body  is  used  for  the  determination  of  the  critical  points. 

Use  of  Neutral  Bodies.  —  The  method  described  above  for  the  detection  of  the 
thermal  critical  points  is  open  to  the  objection  that  the  rate  of  cooling  or  heating 
of  the  steel  under  observation  is  necessarily  affected  by  any  irregularity  in  the  cooling 
or  heating  of  the  furnace  itself  and  by  other  outside  agencies.  These  disturbing  fac- 
tors introduce  irregularities  in  the  thermal  curves  which  may  render  their  interpreta- 
tion difficult  and  may  indeed  altogether  hide  the  existence  of  critical  points  where 
but  a  very  small  amount  of  heat  is  evolved  or  absorbed.  Again,  on  cooling  for  in- 
stance, when  a  critical  point  is  reached  the  temperature  of  the  metal  is  affected  in 
opposite  directions  (1)  by  the  cooling  furnace  which  has  a  tendency  to  lower  its  tern- 


TIM 
IN  SEC( 

E 
NDS 
76 

70 
60 
50 

40 
30 

20 

to 

0 

LU 
1- 

ra 

ARBOr> 

o 

c 

0* 

-L 

^ 

r 

y 

<^ttti 

I 

b 

5*^-~ 

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t^r 

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T^ 

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_. 



200'llW  MOO*  1050°  1000°  060*  900*  850*  800*  75O*  700°  650*  GOO"  550*  66o°450*46o*350' 
TEMPERATURE. 

Fig.  172.  —  Inverse  rate  curves.     Cooling  of  pure  iron,  low  carbon  steel,  high  carbon 
steel,  and  manganese  steel.     (Roberts-Austen.) 

perature  and  (2)  by  the  evolution  of  heat  occurring  at  the  critical  point,  the  effect  of 
which  is  to  raise  its  temperature.  It  is  evident  that  the  cooling  influence  of  the  fur- 
nace has  a  tendency  to  decrease  the  apparent  magnitude  of  the  critical  point  and 
therefore  to  mask  it. 

The  elimination  of  these  objectionable  influences  should  result  in  sharper  thermal 
curves  and  in  the  detection  of  faint  evolutions  or  absorptions  of  heat.  This  was  ac- 
complished by  Roberts-Austen  through  the  use  of  a  neutral  body  and  double  thermo- 
couple so  connected  that  the  difference  of  temperature  between  the  metal  under 
investigation  and  the  neutral  body  is  recorded,  as  well  as  the  actual  temperature  of 
the  metal.1  If  the  heat  capacities  and  emissivities  of  the  metal  and  of  the  neutral 
piece  were  identical  their  temperature  would  be  exactly  the  same  except  at  the  crit- 


1  For  the  arrangement  of  the  galvanometers,  connections,  etc.,  see  the  description  of  the  Saladin- 
Le  Chatelier-Pellin  instrument,  Chapter  I. 


174  CHAPTER   X  — THE   THERMAL   CRITICAL   POINTS   OF   STEEL 

900      800       700       600       500  900      SOO       700       bOO      SOO 


C  =  0,06 
Mn  =0,31 


O  0,10 
Mn-0.31 


C=O.I5 


\ 


, 53 


C--  0,36 
Mn-0,  52 


1 

I 

V 


C- 0,4-8 


I 


=  0,66 


900°    #00°     ?00°    600°     soo°  Temperatures      9OO'    S'OO'    TOO'     (>oo' 

Fig.  173.  —  Difference  curves.     Cooling  and  heating  of  various  steels.     (Saladin.) 

ical  points,  when  heat  is  evolved  or  absorbed  by  the  metal  while  the  neutral  body  is, 
of  course,  free  from  any  such  thermal  disturbance.  Any  difference  of  temperature 
between  the  two  pieces,  therefore,  would  indicate  a  critical  point.  Since  it  is  generally 
impossible,,  however,  to  use  a  neutral  body  having  exactly  the  same  heat  capacity  as 
that  of  the  metal  under  observation,  it  will  be  apparent  that  there  will  always  be  a 


CHAPTER   X  — THE   THERMAL   CRITICAL   POINTS   OF   STEEL 


175 


difference  between  the  temperatures  of  the  two  pieces,  one  always  lagging  behind  the 
other,  and  that  the  critical  points  will  correspond  to  sudden  increase  in  the  difference 
between  their  respective  temperatures.  Since,  however,  the  critical  points  are  now 
caused  solely  by  abrupt  differences  between  the  temperatures  of  the  two  bodies,  they 
are  freed  from  the  irregularities  mentioned  above  as  well  as  from  the  masking  in- 
fluence of  the  falling  or  rising  temperature  of  the  furnace,  seeing  that  both  pieces  are 
now  equally  affected,  and  the  curves  obtained  should  indicate  more  sharply  and  con- 
spicuously the  existence  of  even  faint  absorptions  or  evolutions  of_heat. 

The  neutral  body  should,  of  course,  be  free  from  any  thermal  transformation 
within  the  range  of  temperature  covered  by  the  experiments.  Platinum,  porcelain, 
clay,  25  per  cent  nickel  steel,  and  (by  the  author)  austenitic  manganese  steel  have 
been  used.  The  plotting  of  the  thermal  curves  when  a  neutral  body  is  used  may  be 


900 


Fig.  174.  —  Difference  curves.     Cooling  and  heating  curves  taken  on  same 
photographic  plate.     (Saladin.) 


done  in  two  different  ways,  (1)  successive  falls  (or  rises)  6  —  10,  0  —  20,  6  —  30  ...  of 
the  temperature  of  the  metal  as  indicated  by  one  of  the  galvanometers  may  be  plotted 
as  ordinates  against  the  corresponding  differences  in  temperature  6  —  6\,  6'  —  d'\, 
6"  -  d'\  ...  of  the  two  cooling  bodies,  as  indicated  by  the  second  galvanometer, 
the  coordinates  in  this  case  being  6  and  6  —  61,  and  the  curve  known  as  a  "difference" 
curve,  or  (2)  according  to  Rosenhain,  successive  falls  of  temperature  may  be  plotted 
as  ordinates  against  the  corresponding  rate  of  cooling  for  each  degree  of  temperature 

e  -  el  tf  -  e'1  e"  -  e\ 

>  >  as  abscissae,  the  coordinates  being  in  this  method  0  and 


(6  ~ 

do 


and  the  curve  known  as  a  "derived  differential"  curve. 


Difference  curves  are  shown  in  Figures  173  to  175.  The  curves  of  Figures  173  and 
174  were  taken  with  a  Saladin-Le  Chatelier-Pellin  instrument.  Figure  173  shows  the 
critical  points  on  heating  and  cooling  of  a  series  of  carbon  steels  containing  from  0.06 
to  0.66  per  cent  carbon,  the  cooling  and  heating  curves  having  been  taken  on  separate 
photographic  plates.  In  Figure  174  is  shown  the  heating  and  cooling  curves  taken  on 
the  same  plate  of  two  steels  containing  respectively  0.36  and  0.46  per  cent  carbon. 


176 


CHAPTER   X  — THE   THERMAL   CRITICAL   POINTS   OF   STEEL 


Fig.  175.  —  Difference  curves.     Cooling  of  a  scries  of  very  pure 
carbon  steels.     (Carpenter  and  Keeling.) 

The  curves  of  Figure  175  are  difference  curves  of  a  series  of  very  pure  carbon 
steels  taken  by  Carpenter  and  Keeling. 

The  purpose  of  Rosenhain's  derived  differential  method  of  plotting  is  to  eliminate 
the  irregularities  from  which  difference  curves  still  suffer  and  which  are  due  chiefly  to 
differences  between  the  heat  capacities  and  emissivities  of  the  sample  and  neutral 


CHAPTER  X  — THE   THERMAL   CRITICAL   POINTS   OF   STEEL 


177 


body  resulting  in  differences  in  their  rates  of  cooling  and  heating.  The  resulting 
curves  cannot,  of  course,  be  autographically  recorded.  They  call  for  the  replotting 
of  the  data  afforded  by  the  difference  (0  vs.  0  -  61)  curves.  Some  of  Carpenter 
and  Keeling's  results  replotted  by  Rosenhain  according  to  his  derived  differential 
method  are  shown  in  Figure  176. 


— J 


-  1.000° 


0  02        /0-05        0 12 


Carbon,    per  cent. 


: 


Fig.  176.  — Derived  differential  curves  of  carbon  steels  replotted  from  the  data  of  Carpenter 

and  Keeling.   (Rosenhain.) 


BSD 


0 


030 


S20 


SIO 


Temperature-time 
curve. 


At 

A0 

Inverse  rale 
curve. 


6-0, 

Difference 
curve. 


Derived  differen- 
tial curve. 


Fig.  177.  —  Different  types  of  cooling  curves.     (Desch.) 

Additional  Illustrations  of  Cooling  Curves.  —  The  different  types  of  cooling 
curves  described  in  the  preceding  pages  are  well  illustrated  in  Figure  177.  These 
curves  were  constructed  from  the  data  given  in  the  following  table,  in  which  each  unit 
in  t  represents  intervals  of  time  of  15  seconds,  6  the  corresponding  temperatures  of 
the  sample,  and  6  —  0i  corresponding  differences  of  temperature  between  the  sample 
and  a  neutral  body  cooling  under  identical  conditions.  The  curve  B,  representing 
the  cooling  curve  of  a  neutral  body  free  from  critical  transformations,  has  been  added 
for  comparison. 


178 


CHAPTER  X  — THE  THERMAL  CRITICAL  POINTS  OF  STEEL 


t 

e 

e  e{ 

t 

0 

0-0, 

5 

850.0° 

8.5° 

18 

829.0° 

11.0° 

6 

848.0 

8.5 

19 

825.0 

8.2 

7 

844.7 

7.5 

19.5 

823.3 

7.3 

8 

842.0 

7.0 

20 

822.2 

6.7 

9 

839.5 

6.3 

21 

821.7 

7.7 

10 

838.5 

7.0 

22 

821.5 

8.5 

11 

838.2 

8.8 

23 

821.3 

9.8 

12 

838.1 

10.2 

24 

821.1 

10.1 

13 

838.0 

12.0 

24.5 

819.0 

9.5 

14 

837.9 

13.6 

25 

815.0 

o.O 

15 

837.5 

15.5 

26 

813.0 

5.0 

16 

836.0 

14.6 

27 

811.6 

4.7 

17 

833.0 

13.0 

Self -Recording  Pyrometers.  —  With  the  use  of  neutral  bodies  self-recording  in- 
struments are  generally  employed.  The  Saladin-Le  Chatelier-Pellin  autographic 
pyrometer  has  been  described  in  Chapter  I.  The  self-recording  may  be  by  means  of 
photographic  plates  or  by  some  other  mechanical  devices.  The  former  method 
calls  for  the  use  of  mirror  galvanometers  sending  a  beam  of  light  upon  the  photo- 
graphic plate  while  in  other  autographic  recorders  needle  galvanometers  are  used. 
The  relative  merits  between  photographic  recorders  and  other  types  are  summed 
up  by  Burgess  as  follows: 

"It  is  evidently  of  great  advantage  to  use  self-recording  apparatus  when  possible, 
and  it  then  becomes  necessary  to  choose  between  the  photographic  type  and  the 
autographic.  The  latter  possesses  the  advantage  that  the  experimenter  may  watch 
any  part  of  the  record,  and  can  therefore  control  the  operation  and  at  any  moment 
vary  the  conditions  affecting  the  experiment;  whereas  with  a  photographic  recording 
apparatus,  as  usually  constructed,  the  observer  does  not  know  whether  or  not  the 
experiment  is  progressing  properly  until  it  is  finished  and  he  has  developed  the  sen- 
sitive plate.  The  manipulation  by  the  photographic  method  is  usually  also  more 
delicate  and  time  consuming  and  the  adjustment  less  sure,  and  the  record  often  re- 
quires further  graphical  interpretation.  The  autographic  method  is  in  general  not 
adapted  for  interpreting  phenomena  taking  place  within  an  interval  of  a  few  seconds, 
so  that  for  very  rapid  cooling  it  is  necessary  to  employ  the  photographic  method.  It 
is  possible  to  construct  the  photographic  recorder  so  as  to  obtain  a  very  considerable 
range  of  speeds  with  the  same  apparatus,  while  it  is  difficult  and  costly  to  construct 
an  autographic  recorder  having  more  than  two  speeds." 

Other  Methods  for  the  Determination  of  the  Critical  Points.  —  The  spontaneous 
evolutions  or  absorptions  of  heat  observed  at  the  critical  points  are  obviously  man- 
ifestations of  transformations  taking  place  within  the  metal  and  it  will  be  seen  in  the 
next  chapter  that  many  other  properties  of  iron  and  steel  undergo  at  these  tempera- 
tures sudden  and  marked  changes.  It  is  obvious,  therefore,  that  the  determination 
of  the  change  taking  place  in  any  one  of  these  properties  in  function  of  the  tempera- 
ture will  afford  a  means  of  detecting  the  existence  and  location  of  the  critical  points. 
The  method  fully  described  in  the  preceding  pages,  and  by  far  the  most  generally 
used,  may  be  termed  the  "  thermal  method."  Other  methods  based  on  the  discon- 


CHAPTER  X  — THE   THERMAL   CRITICAL   POINTS   OF   STEEL  179 

tinuity  in  the  variation  of  certain  properties  occurring  at  the  critical  points  will  now 
be  briefly  described. 

Metallographic  Method  for  the  Determination  of  the  Critical  Points.  —  Howe 
and  A.  G.  Levy  have  used,  with  marked  success,  a  metallographic  method  for  the 
determination  of  the  critical  points,  notably  of  the  point  A3.  The  method  for  Ac3  con- 
sists in  noting  the  temperature  at  which  the  re-absorption  of  ferrite  is  found  to  be  com- 
plete, and  for  Ar3,  the  temperature  at  which  rejection  of  ferrite  begins.  It  will  be 
explained  in  the  next  chapter  that  these  phenomena  correspond  to  the  occurrence 
of  the  Ar3  and  Ac3  points.  The  samples  of  hypo-eutectoid  steels  are  first  heated  to  a 
high  temperature  (1200  to  1450  deg.)  and  slowly  cooled  in  the  furnace  in  order  to 
assemble  the  ferrite  into  coarse  masses.  Specimens  of  this  coarsened  steel  are  then 
heated  to  various  temperatures  below  and  above  the  supposed  position  of  Ac3  hold- 
ing the  temperature  stationary  for  from  30  to  60  minutes.  The  specimens  are  then 
quenched  and  preferably  reheated  to  some  300  or  400  deg.  in  order  to  convert  the  mar- 
tensite  into  troostite  and  thus  secure  greater  contrast  between  the  free  ferrite  and  the 
matrix  into  which  it  is  embedded  and  hence  readier  distinction  of  small  amounts  of 
free  ferrite.  These  structural  changes  are  explained  in  Chapters  XVI  and  XVII. 
The  Ar3  point  is  determined  in  a  similar  manner  by  noting  the  temperature  at  which 
free  ferrite  first  makes  its  appearance  in  cooling  down.  The  successive  heatings 
may  be  10  deg.  apart. 

Calorirnetric  Method  for  the  Determination  of  the  Critical  Points.  —  A.  Meuthen 
(Ferrum  1912,  Vol.  X,  p.  1)  has  determined  by  the  vacuum  method  of  Oberhoffer 
(Bunsen  ice  calorimeter  and  electric  resistance  furnace)  the  quantities  of  heat  evolved 
when  iron  carbon-alloys  cool  through  their  critical  points  and  the  method  may  be 
used  for  the  determination  of  these  points. 

.  Thermo-Electric  Method  for  the  Determination  of  the  Critical  Points.  - 
W.  Broniewski  (Comptes  Rendus  1913,  Vol.  CLVI,  p.  1983)  has  applied  a  thermo- 
electric method  to  the  determination  of  the  critical  points  which  he  considers  at  least 
equal  in  accuracy  to  other  methods.  It  consists  in  determining  the  thermo-electro- 
motive  force  of  the  iron  or  steel  in  relation  to  copper.  The  occurrence  of  discontinuity 
in  the  variations  of  the  electro-motive  force  indicates  the  positions  of  the  critical 
points.  With  0.07  per  cent  carbon  the  point  A2  was  found  at  about  730  deg.  In 
pure  iron  A3  occurred  at  950  deg.  and  in  steel  with  0.07  per  cent  carbon  its  position 
was  850  deg.  With  more  than  0.44  per  cent  carbon  the  points  A2  and  A3  were  merged 
with  the  recalescence  point. 

Melting-Points  Method  for  the  Determination  of  the  Critical  Points.  —  Stead 
has  devised  an  ingenious  method  for  the  determination  of  the  critical  points  Ari 
and  Aci  (Journal  Iron  and  Steel  Institute  1913,  II,  p.  399)  by  which  the  use  of 
pyrometers  may  be  dispensed  with,  the  method  being  based  on  the  known  melting- 
point  of  sodium  chloride  and  of  some  metals  in  the  form  of  wire. 

Magnetic  Method  for  the  Determination  of  the  Critical  Points.  —  It  will  be  ex- 
plained in  the  following  chapter  that  iron-carbon  alloys  lose  their  ferro-magnetism, 
or  at  least  the  bulk  of  it,  on  heating  in  passing  through  the  points  Ac2,  Ac3.2,  or  Ac3.2.i 
as  the  case  may  be,  and  recover  it  in  cooling  at  the  points  Ar2,  Ar3.2,  or  Ar3.2.i.  By  ob- 
serving these  magnetic  transformations  and  noting  the  temperatures  at  which  they 
occur,  the  position  of  the  critical  points  may  be  accurately  determined.  The  author 
has  found  the  following  procedure  very  satisfactory.  A  (Fig.  178)  is  an  electric  re- 
sistance furnace;  DD  two  round  bars  of  iron  (the  author  uses  American  ingot  iron,  a 


180 


CHAPTER   X  — THE   THERMAL   CRITICAL   POINTS   OF   STEEL 


product  nearly  free  from  carbon)  suitably  held  in  position;  C  is  the  piece  of  iron  or 
steel  the  magnetic  critical  points  of  which  are  to  be  determined;  X  is  the  hot  junc- 
tion of  a  thermo-electric  couple  introduced  into  a  hole  drilled  in  the  sample  C ;  E  is  an 
electro  magnet  surrounding  a  portion  of  the  bar  D;  G  is  an  iron  needle  so  balanced 
that  it  is  readily  attracted. by  the  end  H  of  the  composite  bar  DCD  when  the  latter 
is  magnetized.  As  long  as  the  temperature  of  the  piece  C  remains  below  its  Aci  point 
(or  Acs,  or  Acs.2.i  according  to  the  carbon  content)  the  bar  DCD  attracts  the  needle 


Fig.  178.  —  Magnetic  test  for  the  determination  of  the 
thermal  critical  points  in  iron  and  steel. 


or  keeper  G  but  when  the  critical  point  is  reached  the  magnetic  flux  ceases  to  pass 
through  C  and  the  needle  drops.  The  point  on  heating  is  in  this  way  sharply  recorded. 
On  cooling  as  soon  as  the  point  Acs  is  reached  the  steel  recovers  its  magnetism  and 
the  needle  G  is  again  attracted.  The  method  is  sufficiently  delicate  to  detect  the 
A2  point  in  carbonless  iron,  and  in  very  low  carbon  steel. 

Historical.  —  A  brief  historical  sketch  of  the  discovery  of  the  critical  points  of 
iron  and  steel  will  not  be  without  interest.  In  1868  Tschernoff  in  studying  the  har- 
dening of  steel  used  the  notation  A  for  the  temperature  at  which  hardening  by  rapid 
cooling  becomes  suddenly  possible  in  high  carbon  steel.  This  was  the  point  AS^.I. 

In  1869  Gore  noted  that  at  a  dark  red  heat  steel  exhibited  on  cooling  a  spon- 


CHAPTER   X  — THE   THERMAL   CRITICAL   POINTS   OF   STEEL  181 

taneous  dilatation  of  short  duration  followed  by  normal  contraction.  Evidently  the 
point  of  recalescence  Ari  or  Ars.2.i. 

In  1873  Barrett  repeated  Gore's  experiments  and  discovered,  on  heating,  a  mo- 
mentary contraction  at  nearly  the  same  temperature  as  the  dilatation  on  cooling. 
This  was  the  point  Aci  or  Acs.2.i.  He  further  noted  that  this  dilatation  or  contraction 
was  very  feeble  in  iron  (the  Ai  point  in  very  low  carbon  steel)  and  very  marked  in 
hard  steel  (the  point  A3.2.i).  Barrett  also  discovered  the  spontaneous  glow  taking 
place  on  cooling  a  wire  and  gave  it  the  name  of  "recalescence." 

In  1879  Barrus  showed  that  the  increase  of  hardness  resulting  from  quenching 
was  not  gradual  but  sudden,  thus  pointing  to  the  existence  of  a  thermal  critical  point 
(the  AI  point). 

In  1885  Osmond  published  his  discovery  of  the  upper  critical  points  As  and  A2  in 
iron  and  low  carbon  steel  and  gave  the  first  accurate  determination  of  the  position  of 
the  point  A!. 


CHAPTER  XI 

THE  THERMAL   CRITICAL  POINTS   OF   STEEL 

THEIR  CAUSES 

The  thermal  critical  points  described  in  the  preceding  chapter  are  evidently  out- 
ward manifestations  of  internal  transformations  taking  place  spontaneously  at  cer- 
tain critical  temperatures.  We  should  now  inquire  into  the  nature  of  these  trans- 
formations. 

Let  us  remember  that  there  are  but  three  well-known  causes  of  spontaneous  evo- 
lutions of  heat  in  cooling  bodies  and  of  spontaneous  absorptions  on  heating.  These 
are:  (1)  formation  of  chemical  compounds,  a  phenomenon  which  is  almost  always 
accompanied  by  a  spontaneous -evolution  of  heat  (the  heat  of  formation),  and  the 
reverse  phase,  the  dissociation  of  the  compound  with  absorption  of  heat  (the  heat  of 
dissociation),  (2)  changes  of  state,  that  is,  the  passage  of  a  substance  from  the  solid 
to  the  liquid  or  from  the  liquid  to  the  gaseous  state,  or  directly  from  the  solid  to  the 
gaseous  state,  which  changes  are  always  accompanied  by  spontaneous  absorptions  of 
heat,  and  the  opposite  phases  of  the  same  phenomena,  the  passage  of  a  body  from 
the  gaseous  to  the  liquid  or  from  the  liquid  to  the  solid  state,  when  heat  is  evolved 
(the  latent  heat  of  solidification  in  case  of  a  substance  passing  from  the  liquid  to  the 
solid  state,  etc.)  these  evolutions  or  absorptions  of  heat,  as  the  case  may  be,  main- 
taining the  temperature  of  the  substance  constant  while  a  change  of  state  is  in  progress 
as,  for  instance,  during  solidification  or  melting,  (3)  allotropic  or  polymorphic  trans- 
formations which  are  always  accompanied  by  an  evolution  of  heat  when  the  body 
passes  from  one  allotropic  condition  to  another,  and  by  an  absorption  of  heat  when 
it  returns  to  its  first  allotropic  form  or  vice  versa.  The  meaning  of  allotropy  has  been 
discussed  in  Chapter  V. 

Causes  of  the  Upper  Points  A3  and  A2  in  Carbonless  Iron.  —  It  has  been  seen  that 
at  these  points  spontaneous  evolutions  or  absorptions  of  heat  occur  in  chemically 
pure  iron  which,  since  they  are  not  accompanied  by  any  change  of  state  (the  metal 
being  considerably  below  its  solidification  point)  must,  it  seems,  necessarily  indicate 
the  existence  of  iron  under  three  allotropic  forms.  It  has  been  mentioned  in  Chapter 
V  that  the  allotropic  form  stable  above  AS  is  known  as  7  (gamma)  iron,  that  stable 
between  A3  and  A2  as  0  (beta)  iron,  and  the  form  stable  below  A2  as  a  (alpha)  iron. 
The  following  then  takes  place  during  the  cooling  and  heating  of  pure  iron:  as  the 
metal  cools  from  a  high  temperature  when  the  point  Ar3  is  reached,  it  passes  from 
the  gamma  to  the  beta  condition  with  evolution  of  heat,  while  at  Ar2  it  passes  from 
the  beta  to  the  alpha  form  also  with  evolution  of  heat.  On  heating  the  reversals 
take  place,  the  iron  passing  with  absorptions  of  heat  from  the  alpha  to  the  beta  con- 
dition at  Ac2  and  from  the  beta  to  the  gamma  condition  at  Ac3.  That  the  point  A3 
indicates  an  allotropic  transformation  is  universally  admitted,  no  one  doubting  the 

182 


CHAPTER  XI  — THE   THERMAL   CRITICAL   POINTS   OF  STEEL  183 

existence  of  iron  in  at  least  two  allotropic  conditions.  Many  authoritative  writers 
believe  with  Osmond  that  the  point  A2  also  indicates  an  allotropic  transformation, 
and  that  iron,  therefore,  assumes  three  distinct  allotropic  forms,  as  explained  above. 
While  some  writers  have  expressed  doubts  as  to  the  allotropic  character  of  the  point 
A2,  as  will  presently  be  explained,  in  these  pages  iron  will  be  assumed  to  exist  in  three 
allotropic  conditions,  of  which  A3  and  A2  are  the  transformation  points,  this  theory 
being,  in  the  author's  opinion,  the  one  best  supported. 

The  A2  Point  and  Beta  Iron.  —  The  arguments  which  have  been  offered  in  sup- 
port of  the  contention  that  the  point  A2  of  pure  iron  and  of  low  carbon  steel  is  not  an 
allotropic  point  and  that,  consequently,  beta  iron  does  not  exist,  should  be  briefly 
considered. 

(1)  It  was  claimed  by  Benedicks  and  Carpenter  that  the  point  A2  or  at  least  Ac2 
does  not  occur  in  strictly  pure  iron  but  in  view  of  the  results  obtained  by  Dr.  Burgess 
at  the  Bureau  of  Standards,  and  by  others,  the  claim  had  to  be  withdrawn  and  the 
existence  of  A2  as  an  independent  point  is  no  longer  a  debatable  question. 

(2)  The  point  A2,  it  has  been  said,  notably  by  Benedicks  and  H.  Le  Chatelier, 
cannot  be  an  allotropic  point  because  of  the  absence  of  hysteresis  between  the  two 
phases  Ac2  and  Ar2  of  the  transformation.    While  it  may  be  that  hysteresis  has  always 
been  observed  in  allotropic  transformations,  it  does  not  by  any  means  follow  that  its 
absence  indicates  the  non-existence  of  allotropy.     Indeed  the  freezing  and  melting 
of  crystalline  substances  which  may  be  considered  as  major  instances  of  allotropy 
occur  normally  without  hysteresis. 

(3)  The  point  A2,  we  are  told,  cannot  be  an  allotropic  point  because  no  crystal- 
lographic  change  has  ever  been  found  to  occur  at  that  point.    The  argument  is  valid 
only  in  case  it  is  accepted  that  an  allotropic  transformation  necessarily  implies  a 
crystalline  change,  a  definition  of  allotropy  which  is  far  from  being  generally  en- 
tertained.    It  is  quite  possible,  moreover,  that   a  slight  crystallographic  change 
occurs  at  A2  which  has  so  far  eluded  observation. 

(4)  The  absence  of  dilatation  as  iron  cools  through  its  Ar2  point  reported  by  some 
(Charpy  and  Grenet,  and  Benedicks)  has  been  claimed  to  preclude  the  existence  of 
allotropy.    This  argument  is  based  on  the  arbitrary  view  that  an  allotropic  trans- 
formation must  necessarily  be  accompanied  by  dilatation  on  cooling  and  contraction 
on  heating.    Some  investigators,  moreover,  believe  to  have  detected  a  dilatation  at 
Ar2,  while  even  the  results  'of  Benedicks'  careful  experiments  do  not  conclusively 
prove  the  absence  of  dilatation. 

(5)  While  admitting  that  a  marked  transformation  takes  place  at  A2  in  the  mag- 
netic properties  of  iron,  it  has  been  argued  that  it  was  not  of  an  allotropic  character 
(H.  Le  Chatelier,  Weiss,  Honda,  Benedicks,  and  others)  (a)  because  it  was  not  suffi- 
ciently sudden  to  imply  discontinuity,  and  (6)  because  in  general  magnetic  trans- 
formations of  that  kind  are  not  allotropic.    Ac2  we  are  told  marks  the  end  of  a  pro- 
gressive transformation  having  its  starting  point  at  a  considerably  lower  temperature, 
while  Ar2  marks  the  beginning  of  the  reverse  change.     Starting  and  end  points  of 
progressive  transformations,  however,  are  not  accompanied  by  sudden  evolution  or 
absorption  of  heat;  slight  changes  of  direction  only  should  occur  in  the  thermal 
curves.     While  it  is  apparently  true  that  the  magnetic  transformation  starts  at  a 
temperature  considerably  below  A2,  the  very  existence  of  that  point  with  its  notable 
heat  evolution  indicates  a  sharp  discontinuity  in  the  progressive-ness  of  the  transforma- 
tion and  hence  suggests  allotropy;  or,  differently  expressed,  iron  on  being  heated  from 


184  CHAPTER   XI  — THE   THERMAL   CRITICAL   POINTS   OF   STEEL 

atmospheric  temperature  to  just  below  its  Ac2  point  loses  progressively  possibly  50 
per  cent  of  its  ferro-magnetism,  while  in  passing  through  its  Ac2  point  it  loses  abruptly 
the  remaining  50  per  cent.  It  is  unjustifiable  to  describe  the  phenomenon  as  merely 
a  progressive  transformation. 

The  recent  theories  put  forward  to  show  that  the  loss  of  ferro-magnetisrn  suffered 
by  some  metals  on  being  heated  should  not  be  regarded  as  allotropic  transformations 
are  purely  arbitrary  and  speculative.  It  remains  more  consistent  to  regard  the  mag- 
netic and  non-magnetic  conditions  of  the  same  substance  as  two  allotropic  states  of 
that  substance. 

(6)  It  has  been  contended  that  no  discontinuity  in  any  of  the  properties  of  iron 
has  ever  been  proved  to  take  place  at  A2  and  that,  therefore,  the  transformation  can- 
not be  an  allotropic  one.     To  this  it  may  be  answered  (a)  that  the  existence  of  a 
marked  thermal  critical  point  indicates  a  sharp  discontinuity  in  the  internal  energy 
of  iron,  and  that  as  an  evidence  of  allotropy  it  is  at  least  as  conclusive  as  a  discon- 
tinuity in  any  other  property,  (b)  that  discontinuity  undoubtedly  exists  at  A2  in  the 
magnetic  properties  of  iron,  and  (c)  that  several  investigators  have  noted  a  discon- 
tinuity in  the  dilatation  of  the  metal  (Rosenhain  and  Humfrey,  although  denied  by 
others),  in  its  tensile  strength  (Rosenhain  and  Humfrey),  in  its  specific  heat  (Weiss 
and  Beck,  and  Meuthen)  and  in  its  electrical  resistance  (G.  K.  Burgess  and  I.  N. 
Kelberg).    In  regard  to  the  latter  property  Burgess  and  Kelberg  write: 

"The  exact  determination  of  the  variation  of  the  electrical  resistance  of  pure 
iron  (99.98)  in  terms  of  temperature  has  been  made  over  the  range  0  to  950  deg.  C., 
particular  attention  being  given  to  the  form  of  the  curve  over  the  A2  and  A3  critical 
ranges  .  .  .  No  anomalies  are  found  in  the  resistance  of  iron  until  the  A2  region  is 
approached,  and  at  A2  there  is  an  inflection  at  757  deg.  C.  in  the  resistance  tempera- 
ture curve  shown  as  a  sharp  cusp  in  the  temperature  coefficient.  At  Ac3  the  resist- 
ance of  iron  falls  abruptly  by  some  0.005  of  its  value,,  which  is  recovered  within  a  25 
deg.  interval,  and  above  Ac3  it  increases  greatly  again.  On  cooling,  the  Ar3  is  accom- 
panied by  slight  increases  of  resistance  with  falling  temperature.  Ac3  and  Ar3  begin 
at  the  same  temperature,  894  deg.  C.,  and  each  extends  over  a  temperature  interval 
of  25  degrees.  These  resistance  measurements  show  that  A2  is  a  strictly  reversible 
transformation  and  A3  is  a  transformation  taking  place  at  a  higher  temperature  on 
heating  than  on  cooling.  These  experiments  are  in  agreement  with  the  thermal  ob- 
servations previously  recorded  in  Scientific  Paper  No.  213. 

Whether  or  not  either  or  both  of  these  critical  ranges  A2  and  A3  are  to  be  consid- 
ered allotropic  points  will  depend  upon  the  definition  of  allotropy,  about  which  there 
does  not  appear  to  be  agreement." 

(7)  To  explain  the  existence  of  A2  in  impure  iron  without  having  recourse  to  al- 
lotropy, Benedicks  attempted  to  show,  through  ingenious  speculations,  that  the  point 
Ar2  indicated  the  end  of  the  Ar3  transformation,  some  gamma  iron  remaining  untrans- 
formed  below  Ar3  owing  to  the  presence  of  impurities,  the  phenomenon  being  one  of 
supercooling,  and  the  resulting  equilibrium  therefore  metastable.     What  had  been 
hitherto  called  beta  iron  was  in  reality,  then,  a  solid  solution  of  gamma  iron  in  alpha 
iron.    As  this  highly  speculative  hypothesis  demands  the  absence  of  A2  (at  least  of 
Ac2)  in  strictly  pure  iron,  it  may  be  considered  as  untenable.    Benedicks,  however, 
facing  this  difficulty  attempted  to  explain  that  unless  the  cooling  be  very  slow  the 
point  Ar2  might  still  occur  in  pure  iron  without  disproving  his  theory  and  likewise 
Ac2,  on  quick  heating.    He  admits,  however,  that  the  intensity  of  A2  should  gradually 


CHAPTER  XI  — THE   THERMAL   CRITICAL   POINTS   OF   STEEL  185 

decrease  as  the  purity  of  the  iron  increases.  The  facts  are  that  both  Ac2  and  Ar2 
occur  sharply  in  the  purest  irons  obtainable  on  very  slow  heating  and  cooling  and 
that  there  are  "no  indications  of  their  decreasing  in  intensity  in  the  purer  metals. 
Benedicks'  attempt  to  replace  a  theory  simple  in  its  conception  and  well  supported 
by  experimental  evidences  by  a  product  of  intellectual  gymnastics  has  failed.  Os- 
mond's theory  originated  and  grew  in  the  laboratory,  in  close  harmony  with  nature, 
while  Benedicks'  theory  was  evolved  in  the  quiet  of  one's  study. 

From  the  foregoing  considerations  the  following  conclusions  appear  warranted: 
(1)  A2  is  an  independent  thermal  point  and  is  a  manifestation  of  a  transformation 
taking  place  spontaneously  in  iron  and  low  carbon  steel,  (2)  A2  can  in  no  way  be  re- 
garded as  the  end  point  or  starting  point  of  the  As  transformation,  and  (3)  whether 
A2  marks  or  not  an  allotropic  transformation  depends  on  our  definition  of  allotropy. 
If  it  be  insisted  that  allotropy  necessarily  implies  a  change  of  crystalline  forms,  then 
it  may  be  argued  that  A2  is  not  an  allotropic  point.  If,  on  the  contrary,  it  is  considered 
that  a  discontinuity  in  the  internal  energy  of  iron,  made  evident  by  spontaneous 
evolution  or  absorption  of  heat,  is  the  criterion  by  which  to  judge  of  allotropy,  then 
we  are  justified  in  believing  in  the  existence  of  beta  iron.  This  view  is  strengthened 
by  the  undoubted  existence  of  a  discontinuity  in  the  magnetic  properties  of  iron  at 
the  A2  point,  and  of  discontinuities  observed  by  some  investigators  in  other  prop- 
erties. 

Causes  of  the  Upper  Critical  Points  A3  and  A2  in  Low  Carbon  Steel.  —  As  might 
be  expected  the  points  A3  and  A2  occurring  in  very  low  carbon  steel  also  indicate 
allotropic  changes  in  the  metal.  According  to  most  writers,  however,  only  that  por- 
tion of  the  metal  which  at  ordinary  temperature  exists  as  free  ferrite  is  here  affected. 
To  make  this  matter  clear  it  will  be  necessary  to  anticipate  somewhat  our  subject 
while  the  diagram  shown  in  Figure  179  will  be  useful. 

In  this  diagram  the  three  critical  points  of  steel  containing  0.20  per  cent  carbon, 
As,  A2,  and  AI  as  well  as  its  solidification  point  and  atmospheric  temperature  are 
represented  by  parallel  lines  drawn  at  suitable  intervals  in  the  scale  of  temperature. 
The  metal  is  represented  by  the  rectangular  area  A  BCD.  The  diagram  illustrates 
the  following  facts  later  to  be  discussed  at  greater  length:  (1)  in  the  molten  condi- 
tion steel  is  considered  to  be  a  liquid  solution  of  iron  and  carbon,  (2)  on  reaching  its 
solidification  point  the  metal  is  converted  into  a  solid  solution  of  gamma  iron  and 
carbon  known  as  austenite,  (3)  upon  reaching  Ar3  some  ferrite  begins  to  be  set  free, 
(4)  the  ferrite  as  it  is  set  free  assumes  the  beta  state,  this  liberation  of  ferrite  and  its 
allotropic  transformation  being  probably  one  and  the  same  phenomenon,  (5)  the 
formation  of  free  ferrite  continues  as  the  steel  cools  from  Ar3  to  Ari,  EFG  represent- 
ing the  ferrite  thus  liberated,  (6)  on  reaching  the  point  Ar2  the  ferrite  liberated  be- 
tween Ar3  and  Ar2,  ML  in  the  diagram,  passes  from  the  beta  to  the  alpha  condition, 
(7)  the  ferrite  liberated  between  Ar2  and  Ari,  LNF  in  the  diagram,  assumes  the  alpha 
condition  (according  to  some  writers)  without  passing  by  the  beta  condition,  while  in 
the  opinion  of  others  the  beta  condition  is  assumed  but  is  immediately  followed  by 
the  alpha  state,  (8)  while  ferrite  is  being  set  free,  the  balance  of  the  steel,  EKIF, 
(according  to  most  writers)  preserves  its  condition  of  solid  solution,  gamma  iron 
plus  carbon,  (9)  upon  reaching  the  point  Ari  the  residual  solid  solution,  FI  is  cpn- 
verted  bodily  into  pearlite,  (10)  from  Ari  down  to  atmospheric  temperature  no  fur- 
ther structural  change  takes  place,  the  steel  being  finally  made  up  of  BH  =  GF  per 
cent  ferrite  and  of  HC  =  FI  per  cent  pearlite. 


186 


CHAPTER  XI  — THE   THERMAL   CRITICAL   POINTS   OF   STEEL 


On 'heating  the  opposite  changes  take  place:  (1)  at  Aci  transformation  of  FI 
pearlite  into  FI  solid  solution  (gamma  iron  +  carbon  =  austenite),  while  this  solid 
solution  begins  immediately  to  assimilate  some  of  the  free  ferrite,  Avhich  as  it  is  as- 


free    Scfa  _ 


A/pha_ 


B 


D 


Liquid    Solution    of 
/f~on    one/   Car-h&n 


Sa//d/f/'cafian 


/ic/  -Sa/ution   of 
'          and 
Co/- ban  CAc/stsnite} 


So//c/   *5  o/vf/on 
(Ausfen/'fe ) 


So/uf/on 


-A, 


•  Pearti fe 


fi 


Fig.  179.  —  Diagram  depicting  structural  changes  in  0.20  per  cent  carbon  steel 
as  it  cools  slowly  from  the  molten  condition  to  atmospheric  temperature. 

similated  passes  to  the  gamma  condition,  (2)  between  AI  and  A3  absorption  of  free 
ferrite  continues,  being  completed  at  Ac3,  (3)  on  reaching  the  point  Ac2  the  ferrite, 
ML  in  the  diagram,  which  has  not  been  absorbed  between  Act  and  Ac2  now  passes  to 


CHAPTKR   XI  —  THE   THERMAL   CRITICAL  POINTS   OF  STEEL  187 

the  beta  condition.  This  diagram  depicts  accurately  the  generally  accepted  views  in 
regard  to  the  meaning  of  the  critical  points.  If  these  views  are  correct  several  inter- 
esting inferences  may  be  drawn  as  to  the  relative  intensities  of  the  critical  points. 
The  point  A3  in  low  carbon  steel  does  not  indicate  a  complete  transformation,  as  too 
often  loosely  stated,  but  merely  the  beginning,  at  Ar3,  or  the  end,  at  Ac3,  of  a  trans- 
formation extending  over  a  considerable  range  of  temperature,  i.e.  from  Ai  to  A3. 
Theoretically,  therefore,  it  would  seem  as  if  the  point  A3  must  correspond  to  a  mere 
change  of  direction  in  cooling  and  heating  curves  rather  than  to  well-marked  jogs. 
The  fact  that  a  decided  jog  marks  the  point  Ar3  in  very  low  carFon  steel  might  be 
ascribed  to  hysteresis,  the  metal  cooling  to  a  temperature  below  that  at  which  the 
A3  change  is  due  so  that  when  the  transformation  begins  to  take  place  it  does  so  with 
added  intensity,  hence  the  jog.  The  jog  corresponding  to  the  Ac3  point  of  very  low 
carbon  steel  is  not  so  readily  explained.  But  is  not  this  jog  much  less  pronounced 
than  the  one  corresponding  to  Ar3?  The  point  Ar2  marks  (1)  a  complete  transforma- 
tion, namely,  the  passage  from  the  beta  to  the  alpha  state  of  the  free  ferrite  liberated 
between  Ar3  and  Ar2  and  (2)  the  beginning  of  a  transformation,  namely,  the  passage 
to  the  alpha  condition  of  the  ferrite  which  continues  to  be  liberated  between  Ar2  and 
Ai'i.  Because  of  the  complete  transformation  implied  by  the  point  A2  we  readily 
understand  that  it  should  correspond  to  a  jog  both  in  the  heating  and  cooling  curves, 
and  since  Ar2  is  due  chiefly  to  the  allotropic  transformation  of  the  ferrite  liberated 
between  Ar3  and  Ar2,  we  readily  understand  why  it  should  occur  at  nearly  the  same 
temperature  regardless  of  the  carbon  content.  In  the  light  of  what  precedes,  how- 
ever, the  point  A3  in  steel  instead  of  being  regarded  as  the  manifestation  of  trans- 
formations occurring  and  completing  themselves  at  a  certain  temperature,  in  reality 
indicates  the  beginning  or  end  of  transformations  extending  over  a  considerable 
range  of  temperature,  namely,  from  A3  to  AI. 

Cause  of  the  Point  A3.2.  —  It  has  been  seen  that  the  point  A3.2  is  apparently  a 
merging  of  the  points  A3  and  A2  of  lower  carbon  steel  and  it  seems  natural  to  infer 
that  the  transformations  which  these  points  indicate,  namely  the  two  allotropic 
changes,  are  here  likewise  merged,  that  is,  that  they  now  take  place  at  the  same  tem- 
perature. In  other  words  that  when  the  point  Ar3.2  is  reached  on  cooling  the  iron 
passes  from  the  gamma  to  the  beta  and  then  immediately  to  the  alpha  state,  the 
heat  evolved  being  due  to  this  double  allotropic  transformation.  Some  writers  have 
claimed,  however,  that  at  the  point  Ar3.2  the  iron  passes  directly  from  the  gamma  to 
the  alpha  condition,  the  change  of  gamma  to  beta  being  suppressed  in  steel  contain- 
ing over  0.35  per  cent  carbon  or  thereabout.  If  such  hypothesis  were  true  it  would 
have  some  important  bearing  upon  the  probable  theory  of  the  hardening  of  steel  as 
explained  in  another  chapter.  In  the  author's  opinion  the  more  generally  accepted 
view  is  better  supported  by  experimental  facts  and  other  evidences  and  in  these 
chapters  the  point  A3.2  will  be  considered  as  implying  a  double  allotropic  change. 
Most  metallographists  believe  that  like  the  independent  points  A3  and  A2  the  double 
point  A3.2  is  the  result  of  allotropic  changes  affecting  the  free  ferrite  only. 

This  setting  free  and  allotropic  transformation  of  ferrite  is  depicted  diagrammati- 
cally  in  Figure  180  in  the  case  of  steel  containing  0.60  per  cent  carbon  and  having, 
therefore,  the  two  critical  points  A3.2  and  At.  EOF  indicates  the  gradual  liberation 
of  ferrite  and  its  conversion  to  the  alpha  state  as  the  metal  cools  from  Ar35  to  Arb 
the  steel,  after  complete  cooling,  being  made  up  of  BH  =  GF  per  cent  ferrite  and 
HC  =  FI  per  cent  pearlite. 


188 


CHAPTER   XI  — THE   THERMAL   CRITICAL   POINTS   OF   STEEL 


If,  as  generally  stated,  the  allotropic  transformation  of  which  A3.2  is  a  manifesta- 
tion affects  only  the  free  (pro-eutectoid)  ferrite,  the  intensity  of  the  point  A3.2  must 
decrease  rapidly  with  decreasing  pro-eutectoid  ferrite,  i.e.  as  the  eutectoid  composi- 


iron  and  car-ban 


So/id    solution 

iron  and 
cc/r-bon  (Austens  fe) 


Temperature 


Fig.  180.  —  Diagram  depicting  structural  changes  in  0.60  per  cent  carbon  steel 
as  it  cools  slowly  from  the  molten  condition  to  atmospheric  temperature. 

tion  is  approached,  and  this  point  must  vanish  altogether  as  it  meets  the  point  AI 
(see  Chapter  X,  Fig.  169),  from  which  it  further  follows  that  the  single  point  of  eutec- 
toid steel  is  not  in  reality  a  triple  point  as  the  notation  A3.2.i  would  imply,  resulting 


CHAPTER  XI  —  THE   THERMAL   CRITICAL   POINTS   OF   STEEL  189 

from  the  merging  of  AI  and  A3.2,  but  that  on  the  contrary  it  remains  a  single  point, 
being  merely  the  continuation  of  the  AI  point  of  hypo-eutectoid  steel. 

Cause  of  the  Point  AI.  —  It  has  been  seen  that  the  point  AI  does  not  occur  in 
carbonless  iron,  only  feebly  in  iron  containing  little  carbon,  and  with  increased  in- 
tensity as  the  carbon  increases  to  the  eutectoid  point.  The  conclusion  seems  irre- 
sistible that  the  point  AI  must  be  closely  related  to  the  carbon,  that  it  must  indicate 
a  sudden  change  in  its  condition.  If  steel  be  rapidly  cooled  from  above  the  point 
Aci  and  then  treated  with  certain  dilute  acids,  practically  all  the  Carbon  escapes  as 
hydrocarbons,  whereas  the  same  steel  after  slow  cooling  through  Ari  when  similarly 
treated  yields  a  carbonaceous  residue,  which  upon  being  analyzed  is  found  to  consist 
of  the  carbide  Fe3C.  It  is  assumed  that  upon  quick  cooling  we  retain  the  carbon, 
partially  at  least,  in  the  form  in  which  it  normally  exists  above  AI,  and  seeing  that 
when  subjected  to  a  similar  treatment  this  carbon  behaves  so  very  differently  from 
the  carbon  of  slowly  cooled  steel,  the  conclusion  is  very  logical  that  carbon  exists 
above  AI  in  a  different  condition  from  that  normal  below  AI.  Above  AI  it  is  called 
"hardening"  carbon,  below  AI  "cement"  carbon.  On  heating  steel  past  the  point 
Aci  the  carbon  changes  from  the  cement  to  the  hardening  condition,  and  vice  versa 
on  cooling  at  Ari  from  the  hardening  to  the  cement  condition.  It  is,  moreover,  gen- 
erally believed  that  this  hardening  carbon  is  carbon  in  solid  solution  in  the  iron.  If 
it  be  so  the  heat  evolved  at  Ari  is  clearly  in  part  at  least  the  heat  of  formation  of 
the  carbide  Fe3C  and  the  heat  absorbed  at  Aci  clearly  the  heat  of  dissociation  of  that 
carbide  which  now  is  resolved  again  into  its  elements  according  to  the  reversible 

reacti°n 


The  intensity  of  the  AI  point  should  then  increase  as  the  carbon  increases  or, 
rather,  as  the  amount  of  pearlite  increases,  and  should  be  maximum,  therefore,  at  the 
eutectoid  point  as  indicated  in  Figure  169,  Chapter  X.  With  higher  carbon  con- 
tent it  diminishes  slightly  because  the  free  (pro-eutectoid)  cementite  which  is  now 
present  takes  no  part  in  the  transformation  occurring  at  Ari,  having  been  formed  at 
a  higher  temperature,  namely,  at  Arcm,  as  later  explained. 

It  would  seem  that  the  cause  of  the  AI  point,  i.e.  the  point  of  recalescence,  is  in 
this  way  explained  in  a  perfectly  satisfactory  manner.  The  correctness  of  this  theory 
appears  to  be  further  supported  by  microscopical  evidences  which  reveal  the  presence 
of  Fe3C  in  slowly  cooled  steel  while  pointing  to  the  probable  absence  of  it  in  suddenly 
cooled  steel. 

In  recent  years,  however,  it  has  seemed  more  and  more  probable  to  students  of 
metallography  that  it  is  not  carbon  in  its  elementary  state  which  is  dissolved  in 
iron  at  a  high  temperature,  but  rather  the  carbide  Fe3C  itself  and  that  the  difference 
between  the  behavior  of  the  carbon  in  hardened  steel  and  in  slowly  cooled  steel  might 
well  be  satisfactorily  accounted  for  on  the  ground  that  in  hardened  steel  Fe3C  is  dis- 
solved in  iron  and  in  that  form  is  much  more  readily  acted  upon  by  acids,  being  there- 
by converted  into  hydrocarbons,  whereas  Fe3C  when  in  the  free  crystallized  condi- 
tion, as  in  slowly  cooled  steel,  resists  the  action  of  the  acids  and  remains  undissolved. 
If  it  is  Fe3C  and  not  C  which  is  dissolved  in  iron  above  the  critical  range,  it  is  evident 
that  the  point  Ari  cannot  be  caused  by  the  formation  of  Fe3C.  But  it  may  well  be 
due  to  the  crystallization  or  falling  out  of  solution  of  Fe3C.  To  be  sure  this  is  a  fall- 
ing out  of  a  solid  solution,  but  cannot  we  conceive  that  the  falling  out  of  a  constit- 
uent of  a  solid  solution  is  accompanied  by  an  evolution  of  heat  even  if  it  does  not 


|«U)  til  \rrii;     \l        Till:    TIIKIiMU.    CKl'I'ICAL    1'OIX'I'S    Oh'    STKK1, 

implx  a  change  df  state?  In  other  words  is  il  not  possible,  or  ex  en  probable,  lhal 
u  \  •.(  a  Hi  :il  i.  Hi  in  I  ho  solid  stair  I  •  accompanied  l>y  an  ex  olulion  of  lira  I'.'  Surely  this 
crystalli/.alion  implies  an  allolropie  or  al  Irasl  a  polymorphic  I  ransl'oniial  ion  and 
arc  mil  snrh  transformations  always  accompanied  by  heat  evolutions'.' 

The  author  ol'l'rrs  llirse  thoughts  as  possibly  worthy  of  allrntion  anil  as  a  possible 
explanation  of  the  evolution  of  lieal  at  Ar,  if  \ve  assiiine  llial  l'V;1t  '  and  not  ( ',  as  il 
ni'\\  seems  so  probable,  is  dissolved  in  iron  al>ovr  lhal  poinl. 

The  I'oint  A,  an  Allotropir  Point  \h>  I    unlers  describe  the  point    A,  as  piirelx 

a  carbon  point,  lhal  is,  a  mamfeslal  ion  of  a  change  affect  ing  I  hr  condilion  of  the 
carbon  only  as  explained  in  the  foregoing  pages.  These  same  wrilrrs,  ho\\ever.  as- 
sert lhal  Ihr  npprr  rritical  points,  A;  and  A-  in  low  carbon  steel  or  V;  •  in  higher  car 
bon  slrrl.  affect  only  the  condilion  of  the  free  i  pro  enlivloidl  ferrile.  In  this  I  hex 
are  inconsistent,  for  if  the  upper  point  or  points  indicale  allot  ropir  transformation  ot 
Ihr  free  frrrite  onlx  then  (he  lo\\er  poinl  A,  is  decided!)  an  allot ropic  poinl  seeing 
thill  it  corresponds  to  allot  ropie  transform.'!!  ions  of  the  pearlilr-frrritr  and  thai  in 
sirel  containing  more  than  some  0.10  per  cent  carbon  ihere  is  move  pearlite  ferrile 
than  free  ferritr.  In  other  \\ords  the  point  A,  is  always  an  allolropie  poinl  indicating 
an  allolropie  I  ran-.loniiMtion  of  the  prarlilr-ferrilr  similar  to  the  allotropic  transfor 
million  of  Ihr  free  ferrite  occurring  al  Ihr  upper  points,  and  in  ease  of  sled  with 
morr  than  0,10  per  cent  carbon  ihe  allolropie  change  taking  place  at  AI  atVects  a 
larger  bulk  of  iron  than  the  change  al  A,...  To  make  the  mailer  clear  let  us  consider 
,  I  i:v  I  Sin  ;,  Meet  conl  .'lining  some  O.i'.O  per  cenl  of  carbon  and,  therefore,  made  up 
H,ft or  slo\\  COOling  of  72  per  COU I  ol  pearhle  and  '.'S  per  COH<  of  free  I'ernte.  Tills  steel 
will  contain  about  7'J  \  ;s  »M  prr  cenl  of  pearlilc  ferrilc  represeiiled  by  !•'(>  in 
Ki.uure  ISO.  \Vhrn  the  point  A,  is  reached  this  (\'.\  per  cent  of  iron  is  slill  in  i  hr  ^amma 
condition  (ftOOOrding  lo  the  general  belief1!  and  now  passes  to  the  alpha  condilion 
rilhrr  dirrclly  or  tirsl  assuming  thr  brla  slate.  The  allolropie  character  of  the  point 
\,  is  thcrrforr  rvident.  Indred  H  is  sullicient  lo  account  for  the  heal  evolved  at  Ar, 
or  absorbed  ill  Ac,  wilhoul  the  assistance  of  any  change  occurring  in  ihe  carbon 
condilion,  for  it  is  in  perfect  agreement  with  the  increased  intensity  of  the  poinl  \ 
M  ,  ilu-  carbon  inorotisos  and  \\  nh  il-.  ma  \nnnm  at  I  he  en  I  eel  old  comp«v-n  ion.  since  a-~ 
ihe  carbon  increases  the  amount  of  prarlilr  and  therefore  of  pearlile-ferrile  likrwisr 
itUToasrs. 

Summing  up,  three  reasons  may  be  iiiven  for  ihe  evolution  ot  heal  al   Ar,:  ^P  lor-- 
million  of  the  carbide  I'r.jl '  ba-ed  on  ihr  assumption  that  carbon  as  such  is  dissolved 
m  iron.  (8)  cr\stalli-ation  of  the  carbide  1'e.r  ba-ed  on  the  assumption  that  this  car 
bidr  is  dissolvrd  in  iron  .and  lhal  erysialli.-aiion  not  implying  a  change  of  slate  max 
produce  heat,  and   v:>^  allolropie  transform;!!  ion  of  ihr  iron  prrsenl    in  austrnilr  ot 
eutecloid  composition.      It   seems  probable  that    both   C-M   :«'ul   i,:^   contribute  to  the 
heat  developed  ill   Al'i. 

rr.uhte  Foim.ition  \\haicvei  ditVerrmvs  ol  opinion  max  exist  a-  to  ihe  exact 
CHUSO  01  causos  of  the  evolution  ol  lioal  oorwspoiulinjj  (o  the  poinl  \r  all  agr«c  ihai 
il  is  due  to  the  transformation  of  austcnito  of  euteeloid  composition  vsomet  lines 
called  hardrnitrl  into  pearlite.  i.e.  the  conversion  of  a  solid  solution  into  an  aggre- 
..,,:,  ferrite  plus  comentito)  li  IS  well  to  be*I  m  mind  tlie  ohMfes  m  il;e  coiuluion 
of  the  iron  and  carbon  which  this  transformation  seems  to  imply:  (Jl  passage  of  the 
non  from  the  giimma  lo  (he  beta  condition.  ^  immediaiely  followed  by  its  conver- 
sion into  alpha  iron  or.  according  to  some  wrilrrs,  vl  and  Ln  the  conversion  of  gamma 


CIIAI'TKK    \l        Till'    TIIKK'MAI,   CRITICAL    I'olNTS    OK    STM  I 


I  !H 


iron  directly  inlo  alpha  iron,  skipping  Ihe  liela  stale,  (ii)  Ihe  crvslalli/inj;-  of  alpha 
iron  inlo  parallel  plates  or  lamella',  and  (  In)  Ihe  formal  ion  and  cr\  slalli/.iiiK,  or  (•!/>) 
Ihe  cryslalli/illii'  '>nl\  of  l''e.,(  '  inlo  parallel  plates  alternalinn  \vilh  Ihe  ferrilt-  plates. 


gamm  iron  one/ 
cordon 


ii  clr|iicl  lii(j;  slMirl  nr:il  rhniipys  in   |.L>:,  pn'  ,TII[  r:irl.,m  .sln-l 
as  it  ('(Mils  sluu  l\    I  I'm 1 1  ll Olten  Condition  In  :ilinns|>liri-ic  Irinprralui'r. 

Cause  Of  the  Point  A  „,.         The  point    Ar,  ,„  nmloiililcdly   indicates    (he    I  le^il 
of   I  lie   lilier.'llioii  of   free  eemelilile   in    livper  eiilecloid  sleel   as   if  cools   from    A,.,,,    In 
AlV'i.     This  gradual  formation  of  free  eemelilile  is  well  shown  in  l''inurc  IS!   \\here  il 


192  CHAPTER   XI  — THE   THERMAL   CRITICAL   POINTS   OF   STEEL 

is  represented  by  the  triangle  EFG.  When  the  point  A3.2.i  is  reached  the  residual 
austenite,  now  of  eutectoid  composition,  is  converted  bodily  into  pearlite,  the  steel 
consisting  finally  of  BH  =  GF  per  cent  free  cementite  and  HC  =  FI  per  cent  pearl- 
ite. It  will  be  seen  that  this  uppor  point  of  hyper-eutectoid  steel,  like  the  points  A3 
and  A2  of  hypo-eutectoid  steel,  does  not  indicate  a  complete  transformation  but 
merely  the  beginning  of  a  transformation  covering  a  wide  range  of  temperature, 
namely,  from  Acm  to  A3.2.i.  If  it  corresponds  to  a  jog  rather  than  to  a  mere  change 
of  direction  in  cooling  curves  this  must  be  ascribed  to  hysteresis  and  its  tendency  to 
accentuate  the  beginning  of  a  transformation  as  previously  explained.  The  evolu- 
tion of  heat  corresponding  to  the  liberation  of  free  cementite  may  be  explained  in 
two  ways:  (1)  actual  formation  of  Fe3C,  carbon  and  not  the  carbide  being  in  solution 
above  Arcm  or  (2)  crystallization  or  falling  out  of  solution  of  Fe3C  based  on  the  more 
probable  assumption  that  Fe3C  and  not  C  is  dissolved  by  the  iron  above  Arcm,  and 
on  the  further  assumption  that  this  crystallization  in  the  solid  state,  since  it  evidently 
implies  an  allotropic  or,  at  least,  polymorphic  change,  must  be  accompanied  by  an 
evolution  of  heat. 

Since  in  hyper-eutectoid  steel  containing  even  as  much  as  1.5  per  cent  carbon 
there  is  but  a  small  proportion  of  free  cementite  (some  1 1  per  cent)  the  point  Arcm  is 
caused  by  the  evolution  of  but  a  small  amount  of  heat  and  must  therefore  be  faint. 
Its  intensity,  moreover,  must  decrease  as  the  carbon  decreases  and  the  point  must 
disappear  altogether  as  the  eutectoid  composition  is  reached,  that  is,  just  as  it  meets 
the  single  point  of  eutectoid  steel.  This  is  indicated  in  Figure  169  of  Chapter  X. 

Allotropy  of  Cementite.  —  If  we  believe,  as  most  metallographists  now  do,  that 
Fe3C  and  not  C  forms  a  solid  solution  with  carbon  above  the  point  AI  or  A3.2.i,  it 
follows  that  this  dissolved  Fe3C  crystallizes  or  falls  out  of  solution  at  certain  critical 
temperatures,  namely,  Arcm  for  the  free  cementite  of  hyper-eutectoid  steel  and  AT, 
(or  Ar3.2.i)  for  the  pearlite-cementite  of  all  steels,  and  that  this  crystallization  is  ac- 
companied by  an  evolution  of  heat.  This  falling  out  of  solution  really  implies  a 
spontaneous  change  of  crystalline  form  and  is  therefore  an  evidence  of  polymorphism, 
hence  of  allotropy,  for  if  allotropy  does  not  necessarily  imply  polymorphism,  poly- 
morphism implies  allotropy.  Are  we  not  then  justified  in  believing  that  Fe3C  may 
exist  under  two  allotropic  forms:  (1)  an  allotropic  variety  soluble  in  iron,  which  we 
may  call  gamma  cementite  and  (2)  an  allotropic  variety  insoluble  in  iron,  which  we 
may  call  alpha  cementite,  constituting  the  free  cementite  of  hyper-eutectoid  steel 
and  the  hard  plates  of  pearlite? 

The  fact  that  the  crystallizing  or  falling  out  of  solution  of  free  ferrite  in  hypo- 
eutectoid  steel  implies  an  allotropic  transformation  of  the  liberated  ferrite,  points 
with  force  to  a  similar  transformation  forming  part  of  the  liberation  of  free  cementite 
in  hyper-eutectoid  steel.  In  the  case  of  iron  we  are  able  to  actually  prove  this  allo- 
tropy through  the  cooling  of  very  pure  iron  and  the  testing  of  its  properties  while  in 
the  case  of  cementite  such  direct  proof  is  not  yet  at  hand  because  of  the  difficulty  of 
obtaining  pure  cementite  and  of  testing  it  after  producing  it. 

To  generalize,  it  would  seem  as  if  the  crystallizing  or  falling  out  of  solution  of  a 
substance  at  certain  critical  temperatures  always  implies  a  spontaneous  change  of 
crystalline  form  and,  therefore,  an  allotropic  transformation  of  the  substance  separat- 
ing from  the  solution,  whether  .that  solution  be  liquid  or  solid.  In  the  former  case 
the  falling  out  of  solution  implies  a  change  of  state,  the  separating  substance  passing 
from  the  liquid  to  the  solid  state,  but  does  it  make  it  less  of  an  allotropic  change? 


CHAPTER   XI— THE    THERMAL   CRITICAL   POINTS   OF   STEEL 


193 


Allot ropic  transformations  which  also  imply  changes  of  state  might  be  called  in- 
stances of  major  allotropy  to  distinguish  them  from  those  instances  in  which  changes 
of  internal  energy  are  not  accompanied  by  changes  of  state. 


l-iquicf  so/tjfion 
iron  and  carbon 


5o//cf 

gamma  /ran  and 

carbon  (Ausfenrfe) 


PearJ/fe 


3  C 

I''ig.  1SU.  —  Diagram  depicting  structural  changes  in  eutectoid  steel  as  it  cools 
slowly  from  the  molten  condition  to  atmospheric  temperature. 

Cause  of  the  Point  A3.2.i  in  Eutectoid  Steel.  —  In  the  case  of  eutectoid  steel  the 
solid  solution  (austenite)  is  originally  of  eutectoid  composition  and,  therefore,  on 
cooling  reaches  the  point  Ar3.2.i  without  rejecting  either  ferrite  or  cementite,  hence 


194  CHAPTER  XI  — THE   THERMAL   CRITICAL   POINTS   OF   STEEL 

the  absence  of  upper  points  in  eutectoid  steel.  On  passing  through  the  point  Ar3.2.i 
(Fig.  182)  this  austenite  is  converted  into  pearlite.  Pearlite  contains  87.50  per  cent 
of  ferrite  which  undergoes  allotropic  transformation  at  Ar3.2.i,  hence  the  intense  allo- 
tropic  character  of  this  point.  The  crystallizing  of  cementite  probably  contributes 
also  to  the  heat  evolved  at  Ar3.2.i  whether  it  implies  the  formation  of  Fe3C  or  not 
as  explained  before.  It  has  also  been  argued  that  this  crystallizing  of  cementite 
probably  implies  an  allotropic  transformation,  in  which  case  the  point  A3.2.i  (and  Ai  as 
well)  would  be  solely  an  allotropic  point,  resulting  from  the  simultaneous  allotropic 
transformation  of  both  the  iron  and  the  Fe3C  of  austenite  of  eutectoid  composition. 

Finally  let  us  bear  in  mind  that  notwithstanding  its  notation  the  single  point  of 
eutectoid  steel  does  not  in  fact  result  from  the  merging  of  the  several  points  of  hypo- 
eutectoid  steel,  being  merely  a  continuation  of  the  point  Ai.  This  point  Ai  is  essen- 
tially a  "pearlite"  point  while  the  upper  points  of  hypo-eutectoid  steel  are  "ferrite" 
points  and  the  upper  point  of  hyper-eutectoid  steel  is  a  "cementite"  point. 

Cause  of  the  Point  A3.2.i  in  Hyper-Eutectoid  Steel.  —  The  point  A3.2.i  in  hyper- 
eutectoid  steel  is  of  exactly  the  same  nature  as  the  point  A3.2.i  of  eutectoid  steel  and 
the  point  AI  of  hypo-eutectoid  steel.  It  marks  the  formation  of  pearlite,  bearing  in 
mind  the  various  changes  in  the  condition  of  the  carbon  and  iron  implied  by  that 
formation.  As  the  proportion  of  pearlite  now  decreases  with  increase  of  carbon  the 
intensity  of  the  point  A3.2.i  likewise  diminishes. 

Summary.  —  The  apparent  causes  of  the  thermal  critical  points  of  iron  and  steel 
may  be  briefly  summed  up  as  follows : 

The  point  Ar3  of  carbonless  iron  and  of  steel  containing  less  than  some  0.35  per 
cent  carbon  marks  the  beginning  of  the  liberation  of  ferrite  (which  liberation  con- 
tinues down  to  the  point  An)  and  the  transformation  of  that  ferrite  from  the  gamma 
to  the  beta  condition,  this  setting  free  of  ferrite  and  its  allotropic  transformation 
being  probably  simultaneous. 

The  point  Ar2  of  carbonless  iron  and  of  steel  containing  less  than  some  0.35  per 
cent  carbon  indicates  the  transformation  from  the  beta  to  the  alpha  condition  of  the 
ferrite  liberated  between  Ar3  and  Ar2  and  the  beginning  of  the  passage  of  the  ferrite, 
which  continues  to  be  liberated  as  the  metal  comes  from  Ar2  to  AT,,  from  the  gamma 
to  the  beta  and  then  to  the  alpha  condition  or,  as  some  writers  claim,  directly  from 
the  gamma  to  the  alpha  state. 

The  point  Ar3.2  of  steel  containing  somewhere  between  0.35  and  0.85  per  cent 
carbon  marks  the  beginning  of  the  liberation  of  ferrite  which  takes  place  between 
Ar3.2  and  Ari  and  the  passage  of  that  ferrite  from  the  gamma  to  the  beta  condition 
and  immediately  to  the  alpha  state  or,  according  to  some  writers,  directly  from  the 
gamma  to  the  alpha  condition. 

Since  only  the  formation  and  transformation  of  free  ferrite  is  involved  at  the  points 
A3,  A2,  and  A3.2  these  points  may  properly  be  called  "ferrite"  points. 

The  point  AI  of  hypo-eutectoid  steel,  as  well  as  the  point  A3.2.i  of  eutectoid  and 
hyper-eutectoid  steel,  marks  the  rather  sudden  transformation  of  the  residual  solid 
solution  (austenite),  now  of  eutectoid  composition,  into  pearlite,  bearing  in  mind  the 
changes  in  the  conditions  of  the  iron  and  carbon  which  such  transformation  implies. 
The  points  AI  and  A3.2.i  may  properly  be  called  "pearlite"  points. 

The  point  Arcm  of  hyper-eutectoid  steel  marks  the  beginning  of  the  setting  free 
of  cementite  as  the  metal  cools  from  Arcm  to  Ar3.2.i,  the  liberation  of  cementite  prob- 
ably involving  an  allotropic  change  of  that  constituent  as  previously  explained. 


CHAPTER  XI  — THE   THERMAL   CRITICAL   POINTS   OF   STEEL 


195 


In  Figure  183  an  attempt  has  been  made  at  showing  diagrammatically  the  rela- 
tion between  the  critical  points  of  steel  and  its  structural  composition  after  slow 
cooling.  It  will  be  readily  understood.  The  upper  part  of  the  diagram  shows  the 
location  of  the  critical  points,  the  lower  part  the  structural  composition  in  percent- 
ages of  ferrite,  pearlite,  and  cementite.  Taking,  for  instance,  a  steel  containing  0.25 
per  cent  carbon :  above  A3  at  A  it  is  a  solid  solution  of  carbon  and  iron  (austenite) ; 
on  cooling  through  Ar3  at  B  some  beta  ferrite  is  set  free,  this  liberation  continuing 
from  B  to  C;  on  cooling  through  Ar2  at  C  the  free  beta  ferrite  is  converted  into  alpha 
ferrite  while  additional  alpha  ferrite  forms  as  the  metal  cools  to  Ari,  that  is  from  C  to 
D.  The  ferrite  liberated  on  cooling  from  B  to  D,  that  is  on  cooling  through  the  criti- 
ical  range,  is  represented  in  the  lower  part  of  the  diagram  by  DE  which  is  also  the 
final  percentage  of  free  ferrite  in  the  steel.  As  the  metal  cools  through  its  Ari  point 


//OO' 


900° 

Cr/f/ccr/ 
Range 

70 O' 


-/-Ausfenrte 
Ar 


5o//d     So/uf/on 
/ron    and    Carbon 
fA  usfen/fe) 


Cemenf/fe 
-f-Ausfenrfe 


%C  O 


a.o 


Fig.  183.  —  Diagram  showing  the  relation  between  the  critical  points  and  the  structural 
composition  of  slowly  cooled  steel. 


at  D,  the  residual  austenite,  at  present  of  eutectoid  composition,  is  converted  into 
pearlite,  EF  representing  the  pearlite  here  formed,  i.e.  the  percentage  of  pearlite  in 
the  steel.  The  structural  formation  of  any  steel  can  be  followed  in  the  same  way  in 
this  diagram.  It  will  be  obvious  that  the  vertical  distances  representing  the  per- 
centages of  ferrite,  pearlite,  or  cementite  in  any  steel  may  also  be  regarded  as  propor- 
tional to  the  intensities  of  the  corresponding  critical  points.  For  instance,  the  dis- 
tance ED  may  be  assumed  to  be  proportional  to  the  combined  intensities  of  the 
points  Ar3  and  Ar2  of  a  0.25  per  cent  carbon  steel  and  the  distance  EF  proportional 
to  the  intensity  of  the  Ai  point.  Interpreted  in  this  way  the  diagram  indicates  what 
has  already  been  pointed  out:  (1)  that  the  intensities  of  A3  and  A2  decrease  as  the 
carbon  increases,  these  points  vanishing  on  reaching  the  eutectoid  composition,  (2) 


CHAPTER   XI  — THE   THERMAL   CRITICAL   POINTS   OF   STEEL 


that  the'point  AI,  very  faint  at  first,  increases  rapidly  with  increased  carbon,  becom- 
ing maximum  at  the  eutectoid  point  and  then  decreasing,  and  (3)  that  the  point 
Acm,  always  faint,  increases  slightly  as  the  carbon  increases  above  0.85  per  cent. 


A 


A,- 


G 


7-<^e    A/f>/>o_ 

Fcrr/tc 


r 


D 


S/c/    -Solution  of 
anaf  Cartoon 


3 of/c/tftccjft on    point 


So/id  Solution  of 
•  Go/77/?7O  Iron  and 
Carbon  { Austin  ite 


-A3 


/ron   on 
Cartion  ( A/tar  f 


•So//&    <5o/L//,a/i  of 
-  A  Ipho  /ron  and 
Carbon  iTroostifis 


•A, 


A  fr 


£>        /y   c 

Fig.  184.  —  Diagram  depicting  structural  changes  in  0.20  per  cent  carbon  steel  slowly 
cooled,  assuming  that  the  iron  remaining  in  solution  as  well  as  the  free  iron  (ferrite) 
undergoes  allotropic  changes.  To  bo  compared  with  Figure  180. 

Another  View  of  the  Allotropic  Changes.  —  It  will  be  obvious  from  the  description  of  the 
underlying  causes  of  the  critical  points  given  in  the  foregoing  pages  that,  according  to  the  general 
belief,  iron  must  first  be  freed  from  solid  solution  before  it  can  undergo  any  allotropic  changes  or,  at 


CHAPTER   XI  — THE   THERMAL   CRITICAL   POINTS   OF   STEEL 


197 


least,  its  liberation  from  solution  and  its  allotropic  transformation  take  place  simultaneously,  the 
latter  never  preceding  the  former.  In  1906  the  author  expressed  the  opinion  that  it  was  far  from 
certain  that  its  liberation  from  solution  must  precede,  or  at  least  be  simultaneous  with,  the  allotropic 
changes  affecting  iron  at  certain  critical  temperatures.  He  ventured  to  put  forward,  in  a  tenf alive 


J!»^^^ 


bo 

£ 


way,  the  hypothesis  that  iron  in  solution  might  first  undergo  an  alJotropic  transformation  and  then 
be  expelled  in  its  new  allotropic  form.  This  view  was  not  favorably  received  but,  as  it  has  not  been 
shown  to  be  by  any  means  untenable,  the  author  still  believes  it  worthy  of  record  in  these  pages.  It 
is  evident  that  if  the  allot ropic  transformation  of  iron  from  the  gamma  to  the  beta  and  then  to  the 


198  CHAPTER  XI  — THE  THERMAL  CRITICAL  POINTS  OF  STEEL 

alpha  state  precedes  its  liberation  from  solution,  three  solid  solutions  of  carbon  in  iron  are  formed 
during  the  slow  cooling  of  steel,  namely,  carbon  (or  the  carbide  Fe3C)  dissolved  (1)  in  gamma  iron, 
(2)  in  beta  iron,  and  (3)  in  alpha  iron.  The  first  solid  solution  is  universally  called  austenite  while 
the  hypothesis  leads  almost  irresistibly  to  regarding  the  solid  solution  in  beta  iron  as  martensite  and 
the  solid  solution  in  alpha  iron  as  troostite,  two  constituents  to  be  described  later. 

With  the  assistance  of  the  diagram,  Figure  184,  and  by  comparing  it  with  Figure  179  the  working 
of  the  present  hypothesis  will  be  readily  understood.  In  Figure  184  are  depicted  the  structural 
changes  taking  place  during  the  slow  cooling  of  steel  containing  0.20  per  cent  carbon  and  therefore 
exhibiting  the  three  critical  points  A3,  A2,  and  Ai.  On  cooling  through  the  point  A3  the  solid  solu- 
tion of  carbon  and  gamma  iron  (austenite)  existing  above  As  is  converted  into  a  solid  solution  of 
carbon  in  beta  iron  (martensite?).  In  the  beta  condition,  however,  iron  cannot  be  retained  in  solu- 
tion and  begins  immediately  to  be  liberated,  and  its  liberation  continues  as  the  metal  cools  down 
to  AZ.  Between  As  and  A2  we  have  a  solid  solution  of  beta  iron  decreasing  in  amount  and  increasing 
free  beta  ferrite.  On  cooling  through  Ar2  both  the  free  beta  ferrite  and  the  dissolved  beta  ferrite  pass 
to  the  alpha  state,  giving  rise  to  the  formation  of  free  alpha  ferrite  and  of  a  solution  of  carbon  in  alpha 
iron  (troostite?).  On  cooling  from  Ar2  to  An  additional  alpha  ferrite  is  liberated  while  the  propor- 
tion of  carbon-alpha  iron  solution  decreases  correspondingly.  At  An  the  remaining  solution  has 
become  of  eutectoid  composition  and  is  converted  bodily  into  pearlite,  the  mechanism  of  this  trans- 
formation being  well  understood.  It  will  be  evident  that  in  the  case  of  hypo-eutectoid  steel  having 
but  one  upper  critical  point,  Ar3.2,  in  cooling  through  that  point  the  metal  would  pass  from  the  condi- 
tion of  a  solid  solution  of  carbon  in  gamma  iron  to  that  of  a  solid  solution  in  beta  iron  and  then 
immediately  to  that  of  a  solid  solution  in  alpha  iron,  the  steel  between  Ar3.2  and  An  being  composed 
of  this  solution  in  alpha  iron  (troostite?)  and  of  free  alpha  ferrite.  With  eutectoid  steel  the  following 
changes  would  take  place  as  it  cools  through  its  single  critical  point  Ar3.2.i:  (1)  transformation  of 
gamma  iron  solid  solution  (austenite)  into  beta  iron  solid  solution  (martensite?),  (2)  immediately 
followed  by  the  formation  of  alpha  iron  solid  solution  (troostite?),  (3)  immediately  followed  by 
formation  of  pearlite. 

The  author  thinks  that  the  decreasing  intensities  of  the  points  A3  and  A2  as  the  carbon  content 
increases  is  the  fact  most  difficult  to  reconcile  with  the  hypothesis  just  outlined,  for  if  these  points 
are  due  to  allotropic  transformations  affecting  the  entire  bulk  of  the  steel  their  intensities  should  be 
quite  independent  of  the  amount  of  carbon  present.  To  be  sure,  this  gradual  diminution  of  the 
magnitude  of  the  points  A3  and  A2  as  the  carbon  increases  is  likewise  difficult  of  explanation  in  the 
light  of  the  universally  accepted  hypothesis  that  free  iron  only  can  be  allotropically  transformed,  for 
it  has  been  made  clear  in  the  foregoing  pages  that  these  points  must  indicate  then  the  beginnings  of 
transformations  and  not  transformations  carried  to  completion  at  those  critical  points,  so  that  the 
intensities  of  the  points  should  be  little  affected  by  the  magnitude  of  the  transformations  themselves, 
that  is,  by  the  amount  of  free  ferrite  undergoing  allotropic  transformation  or,  which  is  the  same  thing, 
by  the  percentage  of  carbon  in  the  steel. 

The  view  just  outlined  as  to  the  mechanisms  of  the  allotropic  changes  is  further  depicted  dia- 
grammatically  in  Figure  185,  in  which  the  critical  points  are  represented  as  covering  certain  ranges 
of  temperature  making  it  possible  to  show,  graphically,  the  changes  taking  place  within  these 
ranges.  Taking  an  iron-carbon  alloy  having,  for  instance,  the  composition  a  (some  0.20  per  cent 
carbon),  the  diagram  shows  that  above  Ar3  it  is  made  up  of  aa',  i.e.  of  100  per  cent  austenite;  on  cool- 
ing through  Ar3  it  is  gradually  converted  into  martensite;  between  Ar3  and  Ar2  beta  ferrite  is 
liberated;  in  passing  through  Ar2  the  ••emaining  martensite  is  gradually  converted  into  a  solid  solu- 
tion of  carbon  and  alpha  iron  (troostite?)  while  the  free  beta  ferrite  is  converted  into  free  alpha 
ferrite;  between  Ar2  and  An  additional  alpha  ferrite  is  liberated;  in  cooling  through  An  the  remain- 
ing solid  solution  (troostite?),  now  of  eutectoid  composition,  is  converted  into  pearlite.  The  struc- 
tural changes  occurring  in  steel  having  but  one  upper  critical  point  and  in  steel  of  eutectoid 
composition  are  similarly  depicted.  This  diagram  is  reproduced  from  the  "Journal  of  the  Iron 
and  Steel  Institute,"  No.  IV  for  1906,  Plate  LII. 


CHAPTER  XII 

THE  THERMAL   CRITICAL  POINTS  OF  IRON  AND   STEEL 

THEIR  EFFECTS 

It  has  been  shown  in  preceding  chapters  that  the  thermal  critical  points  of  iron 
and  steel  are  due  chiefly  if  not  wholly  to  allotropic  transformations  of  the  iron.  It  is 
a  well-known  fact  that  when  a  substance  undergoes  an  allotropic  transformation 
many  of  its  properties  undergo  likewise  deep  and  sudden  changes  at  the  critical 
temperatures.  Color,  crystallization,  dilatation,  conductivity  both  for  heat  and  elec- 
tricity, strength,  ductility,  hardness,  specific  gravity  are  properties  frequently  affected 
as  a  body  passes  from  one  allotropic  form  to  another.  We  should  expect,  therefore, 
such  changes  to  take  place,  as  iron  undergoes  its  allotropic  transformations,  if  not  in 
all,  at  least  in  some  of  the  above  properties.  And  it  is  because  such  changes  do  take 
place  that  a  clear  understanding  of  the  occurrence  and  significance  of  the  critical 
points  is  of  much  practical  importance  to  the  iron  and  steel  metallurgist. 

CHANGES  AT  A3 

It  has  been  shown  that  the  point  A3  occurs  in  carbonless  iron  and  in  steel  con- 
taining less  than  some  0.35  per  cent  of  carbon  and  that  it  is  universally  believed  that 
this  point  indicates  an  allotropic  change,  the  iron  passing  from  the  gamma  to  the 
beta  condition,  or  according  to  some  to  the"  alpha  condition,  on  cooling  at  ATS,  and 
vice  versa,  from  the  beta  (or  alpha)  to  the  gamma  condition  on  heating  at  Ac3.  It 
should  be  borne  in  mind,  however,  as  fully  explained  in  Chapter  XI  that  the  general 
belief  is  that  free  ferrite  only  undergoes  this  change.  As  the  metal  cools  past  its  point 
Ar3  the  following  abrupt  changes  in  some  of  its  properties  have  been  noted. 

Dilatation.  —  The  metal,  which  above  the  point  Ar3  was  contracting,  as  is  the 
general  rule  with  all  cooling  bodies,  on  passing  through  the  point  Ar3  undergoes  sud- 
denly a  marked  dilatation,  amounting  to  over  iVoo  of  its  length,  immediately  fol- 
lowed again  by  a  normal  contraction.  Such  dilatation  implies  that  the  change  of 
gamma  into  beta  iron  takes  place  with  augmentation  of  volume,  or  in  other  words 
that  gamma  iron  is  denser,  has  a  higher  specific  gravity  than  beta  iron.  The  dilata- 
tions occurring  at  Ar3  in  the  case  of  steels  containing  respectively  0.05  and  0.15  per 
cent  carbon  are  shown  graphically  in  Figure  186.  On  heating,  at  Ac3  a  spontaneous 
contraction  occurs  of  the  same  magnitude  as  the  dilatation  on  cooling.  Benedicks 
determined  with  great  care  the  dilatation  of  pure  iron  between  700  and  950  deg.  C. 
and  some  of  his  results  are  shown  in  Figure  187.  The  sharp  occurrence  of  a  marked 
dilatation  at  C,  the  A3  point,  will  be  noted.  Had  we  no  other  evidence  of  an  allo- 
tropic transformation  at  this  critical  temperature,  this  sudden  dilatation  taking  place 
as  it  does  in  pure  iron  would  justify  our  belief  in  its  existence. 

199 


200     CHAPTER  XII  — THE  THERMAL  CRITICAL  POINTS  OF  IRON  AND  STKKL 

Electrical  Conductivity.  —  Above  the  point  A3  the  metal  has  an  electrical  resis- 
tance some  ten  times  greater  than  its  resistance  at  ordinary  temperature.  As  it  cools 
from  a  high  temperature  to  the  point  Ar3  there  is  but  a  feeble  decrease  of  its  electrical 
resistance,  but  as  soon  as  Ar3  is  reached  it  begins  abruptly  and  sharply  to  decrease 
and  keeps  on  decreasing  at  a  normal  rate  to  atmospheric  temperature.  At  A3,  there- 
fore, we  have  a  sudden  and  marked  change  in  the  variation  of  the  electrical  conduc- 
tivity corresponding  to  a  sharp  break  in  the  curve  expressing  the  relation  between 
temperature  and  electrical  resistance  as  shown  in  Figure  188.  On  heating,  at  Ac;t  the 
opposite  change  takes  place,  that  is,  the  electrical  resistance  quite  suddenly  ceases  10 


200-         40O°  6OO°  300'          /OCX? /MX)' 

Fig.  186.  —  Dilatation  curves  of  various  carbon  steels. 


increase.  So  marked  and  sudden  a  change  in  a  physical  property  is  in  itself  a  proof 
of  an  allotropic  transformation. 

Crystallization.  —  It  has  been  shown  in  Chapter  V  that  while  gamma  and  beta 
iron  both  crystallize  in  the  cubic  system  (Osmond)  octahedra  are  the  prevailing  form 
of  gamma  iron  while  the  cube  is  the  crystalline  form  of  beta  iron,  and  that  the  trans- 
formation of  gamma  into  beta  iron  includes  a  change  in  the  planes  of  symmetry,  at 
least  of  carburized  iron  (Osmond). 

Tensile  Strength.  —  Rosenhain  and  Humfrey  have  shown  the  existence  of  a  dis- 
tinct discontinuity  in  the  tensile  strength  of  iron  at  the  A3  point. 

Dissolving  Power  for  Carbon.  —  Above  the  point  A3  iron  possesses  dissolving 
power  for  carbon,  while  according  to  some  writers  it  loses  that. power  on  passing- 
through  Ar3;  in  other  words  gamma  iron  can  dissolve  carbon  but  beta  iron  is  deprived 
of  that  power.  It  does  not,  however,  seem,  by  any  means,  proven  that  beta  iron 
cannot  dissolve  carbon,  many  authoritative  workers  holding  the  opposite  view.  This 
question  will  be  discussed  at  greater  length  in  another  chapter. 


CHAPTER  XII  —  THE  THERMAL  CRITICAL  POINTS  OF  IRON  AND  STEEL     201 


mm. 
1500 

600 

^500 
\ 

S 

i 

i. 

x 

C 

N. 

c 

J 

h 

1 

100 

1 

J«7 

tf 

/?// 

*EREM 

'AL   Dl 

.ATATIL 

N  OF  J 

9ON-G 

OLD 

X 

{ 

B 

\ 

C 

^ 

'? 

\ 

a 

80 

O/Mifflfflt 

70 

70'                      700"                        600°                         300"                         /000°C 

TEMPERATURE > 

Fig.  187.  — •  Dilatation  of  pure  iron.     (Benedicks.) 


Structural  Properties.  —  It  has  been  explained  at  length  in  Chapter  XI  that  the 
point  Ar3  corresponds  to  an  abrupt  structural  change,  namely,  the  beginning  of  the 
setting  free  of  ferrite  (Fig.  179,  Chapter  XI). 

Other  Properties.  —  Le  Chatelier  mentions  a  change  in  the  variation  of  the 
thermo-electric  force  and  a  sudden  but  slight  variation  in  magnetic  properties  as 


202    CHAPTER  XII  —  THE  THERMAL  CRITICAL  POINTS  OF  IRON  AND  STEEL 

taking  place  at  A3.    Meuthen  reports  a  discontinuity  in  the  specific  heat  of  iron  be- 
tween 880  and  900  deg.  C.,  that  is  at  the  A3  point. 


CHANGES  AT  A2 

As  a  separate  point  A2  occurs  in  carbonless  iron  and  in  steel  containing  less  than 
some  0.35  per  cent  of  carbon.  The  changes  of  properties  taking  place  at  A2  are  not 
generally  as  abrupt  nor  are  they  as  marked  as  those  occurring  at  the  other  critical 
points.  It  is  precisely  because  of  this  lack  of  sharpness  and  suddenness  that  some 
metallographists  have  questioned  the  accuracy  of  the  view  that  this  point  like  A3 
indicates  an  allotropic  transformation.  Careful  consideration  of  the  evidences  at 
hand  appear  to  show,  however,  that  changes  of  properties  do  occur  at  A2  sufficiently 
marked  and  sudden  to  warrant  the  classification  of  this  point  as  an  allotropic  one. 
The  fact  that  these  changes-are  more  gradual  than  at  the  other  critical  points  is 


| 


O°        2OO°      4OO°      6OO°      GOO"       /OOO°      /2OO° 
Fig.  188.  —  Electrical  resistance  curves  of  iron  and  high  carbon  steel. 

logically  explained  by  Osmond  on  the  ground  that  beta  and  alpha  iron  are  isomor- 
phous,  that  is,  capable  of  forming  solid  solutions  and  that  therefore  the  passage  of 
one  variety  into  the  other  must  necessarily  be  gradual  as  well  as  the  variations  of  the 
properties  of  iron  which  the  transformation  implies. 

Dilatation.  —  According  to  Le  Chatelier,  to  Charpy  and  Grenet,  to  Benedicks, 
and  others,  no  dilatation  takes  place  as  the  metal  cools  past  the  point  Ar2  and  they 
see  in  this  an  indication  that  A2  is  not  an  allotropic  point.  Osmond's  reply  is  that 
the  curves  obtained  by  Charpy  and  Grenet,  for  instance,  do  indicate  a  dilatation  at 
Ar2  which,  however,  the  authors  fail  to  notice  because  the  transformation  not  being 
sudden  the  dilatation  likewise  is  gradual,  whereas  the  authors  were  looking  for  sud- 
den dilatations  only.  Some  of  Benedicks'  careful  measurements  of  the  dilatation  of 
pure  iron  are  shown  graphically  in  Figure  187.  The  contraction  observed  on  heating, 
between  A  and  B,  that  is  during  the  A2  range  is  described  by  him  as  a  "slight,  fully 
continuously  occurring  contraction  (under  heating)  which  entirely  coincides  with  the 
gradual  disappearance  of  the  ferro-magnetism."  Considering,  however,  that  the 
striction  which  clearly  reveals  the  position  of  A2  covers  a  range  of  temperature  not 
exceeding  some  15  deg.,  it  seems  to  point  to  a  discontinuity  in  the  dilatation  of  the 
metal  rather  than  to  a  perfectly  progressive  transformation. 


CHAPTER  XII  —  THE  THERMAL  CRITICAL  POINTS  OF  IRON  AND  STEEL     203 

Magnetic  Properties.  —  Above  the  point  A2  steel  is  non-magnetic,  it  is  not  at- 
tracted by  a  magnet,  but  in  passing  through  Ar2  it  suddenly  becomes  strongly  mag- 
netic. Or,  to  use  physicists  terms,  above  A2  iron  is  para-magnetic,  below  A2  it  is 
ferro-magnetic.  Is  not  this  abrupt  and  momentous  change  alone,  in  the  magnetic 
properties  of  a  metal,  sufficient  proof  of  an  allotropic  transformation?  It  is  true 
that  magnetism  is  not  fully  regained  until  a  considerably  lower  temperature  is  reached, 
probably  some  550  deg.  C.,  according  to  Osmond,  but  the  fact  remains  that  the 
greatest  part  of  the  final  magnetism  of  the  metal  is  abruptly  acquired  as  it  cools  past 
the  point  A2.  What  transformation  other  than  an  allotropic  one  can  satisfactorily 
account  for  this? 

The  relation  between  the  carbon  content  of  steel  and  the  temperatures  at  which 
the  metal  loses  its  magnetism  on  heating  and  regains  it  on  cooling  is  shown  graphically 
in  Figure  189.  The  points  plotted  in  this  diagram  represent  the  average  values  of  a 
great  number  of  determinations  made  by  Madame  Sklodowska  Curie  with  a  series  of 


0.1          OJ  OJ          a* 


Fig.  189.  —  Temperatures  of  magnetic  transformations  of  various  carbon  steels. 


very  pure  carbon  steels.  It  will  be  noticed  that  the  points  of  magnetic  changes  cor- 
respond closely  to  the  thermal  critical  points  A2,  A3.2,  or  A3.2.i.  With  little  carbon 
there  is  but  a  small  gap  between  the  appearance  of  magnetism  on  cooling  and  its  dis- 
appearance on  heating,  because  the  points  Ar2  and  Ac2  occur  at  nearly  the  same 
temperature;  with  0.50  per  cent  carbon  the  magnetic  points  are  lowered  and  so,  like- 
wise, the  point  A3.2  while  the  gap  increases,  this  being  consistent  with  the  greater  gap 
between  Ar3.2  and  Ac3.2;  with  0.84  per  cent  carbon  the  magnetic  points  are  further 
lowered  and  the  gap  between  them  increased  still  more,  this  being  in  harmony  with 
the  location  of  the  point  A3.2.i  which  is  lower  than  A3.2  and  with  the  greater  gap  be- 
tween Ar3.2.i  and  Ac3.2.i. 

Further  experimental  evidences  that  the  points  of  magnetic  transformations  coin- 
cide with  the  thermal  critical  points  are  given  in  the  following  tables  showing  the  re- 
sults of  several  hundred  determinations.  All  the  steels  used  in  connection  with  the 
results  given  in  Table  II  contained  in  the  vicinity  of  one  per  cent  carbon. 

A  magnetic  method  for  the  determination  of  the  A2,  A3.2  or  Aa^.i  points  has  been 
described  in  Chapter  X. 


204     CHAPTER  XII  — THE  THERMAL  CRITICAL  POINTS  OF  IRON  AND  STEEL 


TABLE   I.  —  COMPARISON   OF  THE   MAGNETIC   METHOD   WITH   THE 
ORDINARY   OR   COOLING-CURVE   METHOD.     (BOYLSTON.) 


APPROXIMATE 
CARBON  CONTENT 
OF  STEEL 

METHOD 

Ac,,, 

NUMBER  OF 
TESTS 

A,.,, 

NUMBER  OF 
TESTS 

MEAN 

MAX. 
MlN. 

MEAN 

MAX. 

Mm. 

Magnetic  .... 

773 

'{788 
1  759 

7 

708 

{711 
1706 

7 

1.25% 

Ordinary   .... 

764 

{781 
1764 

4 

718 

{736 
1701 

4 

Ac3.., 

Ar,,.2 

Magnetic  .... 

780 

{790 
1770 

7 

741 

{750 
1734 

7 

0.40% 

Ordinary   .... 

827 

{830 
1822 

3 

754 

{756 
1  753 

3 

Aca 

Ait 

Magnetic  .... 

764 

{772 
I  755 

6 

764 

(771 
1758 

7 

0.15% 

Ordinary   .... 

768 

{772 
1764 

2 

767 

{783 
1748 

3 

TABLE    II. —RESULTS    OBTAINED    BY    STUDENTS    AT    HARVARD    UNIVERSITY 


STEEL 
NUMBER 

METHOD 

Ae,.j.i 

NUMBER  OF 
TESTS 

Arj.j., 

NUMBER  OF 

TESTS 

I 

Magnetic  .... 

753 

72 

679 

75 

Ordinary   .... 

739 

14 

688 

12 

2 

Magnetic  .... 

752 

131 

695 

136 

Ordinary   .... 

750 

13 

695 

17 

Q 

Magnetic  .... 

761 

55 

704 

.->.-> 

Ordinary  .... 

757 

55 

70S 

55 

4 

Magnetic  .... 

762 

50 

700 

50 

Ordinary   .... 

751 

45 

701 

45 

5 

Magnetic  .... 

776 

40 

720 

40 

Ordinary   .... 

760 

30 

727 

30 

Crystallization.  —  The  cube  being  the  crystalline  form  both  of  beta  and  of  alpha 
iron  and  these  two  allotropic  varieties  being  capable  of  dissolving  each  other  in  all 
proportions  (Osmond)  a  crystalline  transformation  at  the  point  A2  is  not  to  be  ex- 
pected. The  crystallography  of  iron  so  far  as  it  has  been  investigated  does  not  re- 
veal the  existence  of  the  point  A2. 

Tensile  Strength.  —  Rosenhain  and  Humfrey  have  shown  the  existence  of  a  dis- 
tinct discontinuity  in  the  tensile  strength  of  iron  at  the  A2  point. 


CHAPTER  XII  —  THE  THERMAL  CRITICAL  POINTS  OF  IRON  AND  STEEL     205 

Dissolving  Power  for  Carbon.  —  Most  metallographists  believe  that  alpha  iron 
does  not  possess  any  dissolving  power  for  carbon,  or  at  least  that  it  is  only  capable  of 
dissolving  a  very  small  amount  of  that  element.  On  the  other  hand,  as  already  men- 
tioned, it  is  far  from  certain  that  beta  iron  is  incapable  of  dissolving  carborv  state- 
ments to  the  contrary  notwithstanding.  It  is  possible,  therefore,  that  on  cooling 
through  Ar2,  iron  loses  its  dissolving  power  for  carbon. 

Structural  Properties.  —  By  referring  to  Figure  179  of  Chapter  XI  it  will  be  seen 
that  there  is  no  apparent  structural  change  connected  with  the  pojnt_A2.  As  the  steel 
cools  past  Ar2  the  liberation  of  ferrite  started  at  Ar3  merely  continues,  to  end  only  at 
An.  Of  course  the  ferrite  liberated  above  Ar2  now  passes  from  the  beta  to  the  alpha 
condition  but  this  allotropic  transformation  does  not  appear  to  include  any  struc- 
tural change. 

Specific  Heat.  —  Weiss  and  Beck  and  Meuthen  have  shown  that  a  sharp  dis- 
continuity occurred  in  the  specific  heat  of  iron  at  the  A2  point. 


CHANGES  AT  A3.2 

It  has  been  shown  that  the  point  A3.2  resulting  from  the  merging  of  AS  and  A2 
occurs,  theoretically  at  least,  in  steels  containing  from  some  0.35  to  0.85  per  cent 
carbon.  As  might  be  expected  the  changes  of  properties  corresponding  to  the  point 
A3.»  are  the  same  as  those  taking  place  in  lower  carbon  steel  at  A3  and  A2.  As  the 
metal  cools  through  Ar3.2  the  following  variations  of  properties  are,  therefore,  noted: 
(1)  a  marked  dilatation,  (2)  a  sudden  decrease  of  electrical  resistance,  (3)  a  gain  of 
magnetism,  (4)  a  discontinuity  in  the  variation  of  the  tensile  strength  and  of  the 
specific  heat,  (5)  a  probable  loss  of  dissolving  power  for  carbon,  (6)  the  beginning  of 
the  liberation  of  alpha  ferrite  (see  Chapter  XI,  Fig.  180). 


CHANGES  AT  AI 

The  point  AI  occurs  in  steel  containing  from  a  mere  trace  to  0.85  per  cent  carbon. 
It  corresponds,  as  explained  in  Chapter  XI,  to  the  transformation  of  the  residual 
austenite  (now  of  eutectoid  composition)  into  pearlite.  This  formation  of  pearlite 
implies  that  the  iron  contained  in  this  residual  austenite  (and  forming  about  85  per 
cent  of  its  bulk)  undergoes  on  cooling  through  An  the  same  allotropic  changes  as 
those  affecting  the  free  ferrite  on  cooling  through  Ar3  and  Ar2  (or  Ar3.2).  It  follows 
from  this  that,  theoretically  at  least,  the  following  sudden  changes  of  properties 
should  be  noted  on  cooling  through  An:  (1)  a  dilatation  increasing  with  the  carbon 
content  and  being  maximum  with  0.85  per  cent  carbon  caused  by  the  allotropic 
transformation  of  the  iron,  (2)  increased  magnetism  because  of  the  transformation  of 
additional  non-magnetic  gamma  iron  into  magnetic  alpha  iron,  (3)  decreased  elec- 
trical resistance  because  of  additional  transformation  of  high  resistance  gamma  iron 
into  low  resistance  alpha  iron,  and  (4)  additional  loss  of  dissolving  power  for  carbon 
because  of  the  formation  of  additional  alpha  iron. 

The  point  AI  is  not  generally  associated  with  critical  variations  of  the  electrical 
and  magnetic  properties  of  steel,  but  on  purely  theoretical  ground  it  does  not  seem 
possible  to  avoid  the  conclusion  that  such  critical  variations  must  exist  provided  of 
course  that  we  are  right  in  assuming  that  in  austenite  of  eutectoid  composition  the 


206     CHAPTER  XII  —  THE  THERMAL  CRITICAL  POINTS  OF  IRON  AND  STEEL 

iron  is  still  in  the  gamma  condition,  that  is,  non-magnetic  and  of  high  electrical  re- 
sistance. Recent  investigations  have  indeed  shown  that  AI  is  distinctly  a  magnetic 
point. 

CHANGES  AT  A3.2.i 

The  point  A3.2.i  occurs  in  eutectoid  and  in  hyper-eutectoid  steel  and  marks  the 
transformation  of  the  austenite  or  eutectoid  steel  or  of  the  residual  austenite  of  hyper- 
eutectoid  steel,  into  pearlite,  as  shown  in  Figures  181  and  182,  Chapter  XI.  In  these 
steels,  however,  no  liberation  of  free  ferrite  occurs  above  the  point  Ar3.2.i  from  which 
it  follows  that  the  totality  of  the  iron  undergoes  its  allotropic  change  or  changes  on 
passing  through  Ar3.2.i.  The  variations  of  the  properties  which  in  hypo-eutectoid 
steel  occur  at  A3,  A2,  and  AI  must  therefore,  in  the  case  of  eutectoid  and  hyper- 
eutectoid  steel  all  take  place  at  the  point  A3.2.i.  These  sudden  changes  of  properties 
are,  on  cooling:  (1)  a  marked  dilatation,  maximum  in  eutectoid  steel  (Fig.  186), 
(2)  a  sudden  decrease  of  electrical  resistance,  (3)  a  sudden  gain  of  magnetism,  (4)  a 
loss  of  dissolving  power  for  carbon. 

Le  Chatelier  mentions  the  fact  that  on  cooling  through  Ari  or  Ar3.2.i  steel  ac- 
quires a  temporary  malleability.  If  a  steel  bar,  for  instance,  of  sufficient  length  be 
held  horizontally  by  one  extremity  while  cooling,  it  at  first  remains  rigid,  but  on 
passing  through  its  point  of  recalescence  it  quite  suddenly  bends.  According  to 
Howe  this  phenomenon  was  first  observed  by  Coffin. 

Structural  Change  at  AI  and  A3.2.i.  —  The  spontaneous  transformation  of  aus- 
tenite of  eutectoid  composition  into  pearlite,  that  is,  of  a  solid  solution  into  an  aggre- 
gate, at  Ari  or  Ar3.2.i  and  the  reverse  transformation,  from  aggregate  to  solid  solu- 
tion, at  Aci  or  Ac3.2.i,  imply  structural  changes  of  momentous  importance  to  the  steel 
metallurgist.  While  these  will  be  dealt  with  at  length  in  another  chapter  it  seems 
proper  to  record  here  their  significance.  These  structural  changes  give  the  key  to  the 
rational  treatment  of  steel.  They  make  possible  the  refining  of  steel  by  heat  treat- 
ment seeing  that  on  heating  steel  through  its  critical  range  we  may  change  it  from 
the  condition  of  a  coarse  aggregate  (a  coarse  structure)  to  the  condition  of  a  fine, 
nearly  amorphous,  solid  solution.  They  also  make  possible  the  hardening  of  steel 
through  sudden  cooling  from  above  the  critical  range  as  will  be  fully  explained  in 
another  chapter. 

CHANGES  AT  Acm 

The  point  Arcm  occurs  in  hyper-eutectoid  steel  and  marks  the  beginning  of  the 
liberation  of  free  cementite  as  the  metal  cools  from  Arcm  to  Ar3.2.i  (see  Fig.  181, 
Chapter  XI).  Except  for  this  structural  change  no  other  marked  changes  of  prop- 
erties have  so  far  been  connected  with  this  point. 

Prevailing  Conditions  Above  and  Below  the  Critical  Range.  —  The  following- 
condensed  statement  of  some  of  the  most  significant  conditions  prevailing  above 
and  below  the  critical  range  of  iron-carbon  alloys  may  be  useful  in  keeping  these 
fundamental  facts  in  mind.  By  critical  range  is,  of  course,  meant  here  the  critical 
points,  both  on  heating  and  cooling,  considered  collectively  and  it  will  be  understood 
that  the  conditions  described  as  existing  above  the  range  change  quite  abruptly  to 
the  conditions  prevailing  below  the  range  as  the  metal  cools  through  the  range,  or 
vice  versa  as  it  is  heated  above  it.  The  references  made  to  the  crystallizing  of  the 


CHAPTER  XII  —  THE  THERMAL  CRITICAL  POINTS  OF  IRON  AND  STEEL     207 

metal,  and  to  the  influence  of  work  both  above  and  below  the  range  will  be  under- 
stood after  reading  the  following  chapters  dealing  with  the  treatment  of  steel. 


CONDITIONS  AND  PROPERTIES  OF  IRON-CARBON  ALLOYS  AND  OF  THEIR 

CONSTITUENTS 


Above  Critical  Range 
Solid  solution  (austenite). 
Hardening  (dissolved)  carbon. 
Gamma  iron. 
Alloys  containing  a  sufficient  amount  of 

carbon  possess  hardening  power. 
Alloys  are  non-magnetic. 
Metal  crystallizes  on  slow  cooling. 
Work  prevents  crystallization. 


Below  Critical  Range 
Aggregate  (ferrite  +  cementite). 
Cement  carbon  (Fe3C). 
Alpha  iron. 
Same  alloys  deprived  of  hardening  power. 

Alloys  are  magnetic. 

Metal  does  not  crystallize  on  slow  cooling. 

Work  distorts  structure. 


Properties  of  Gamma,  Beta,  and  Alpha  Iron.  — -  Tne  various  properties  of  gamma, 
beta,  and  alpha  iron  described  in  the  preceding  pages,  have  been  tabulated  below  as 
well  as  some  other  data  of  interest.  These  are  in  accordance  with  the  views  held  by 
many,  but  the  author  is  well  aware  that  those  entertaining  different  views  may  take 
exception  to  some  of  the  entries. 


GAMMA  IRON 

BETA  IRON 

ALPHA  IRON 

Metallurgical  name 

Austenite 

Bel  a  ferrite 

Ferrite,    alpha   ferrite, 

pcarlite  ferrite 

Solvent  power  for  C  (or 

dissolves   carbon   up   to 

probably  some  but  opin- 

probably    none,      but 

Fe,C) 

1  .7  per  cent  or  Fe3C  up 

ions  differ 

opinions  differ 

to  25.5  per  cent 

Range    of    temperature 

above  A3,  A3.2,  or  A3.2.i  in 

as  free  beta  ferrite  be- 

below A2)  A3.2,  or  AS.M 

in  which  stable 

case    of    pro-eutectoid 

tween  A3  and  A2 

ferrite,     above    At     or 

A3.  2.1  in  case  of  eutec- 

toid  ferrite 

System    of    crystalliza- 

cubic (orthorhombic  ac- 

cubic 

cubic 

tion 

cording   to   Le   Chatc- 

lier) 

Prevailing       crystalline 

octahedra 

cubes 

cubes 

forms 

Other   crystalline   char- 

frequent twinnings 

no  twinnings 

no  twinnings 

acteristics 

Specific  gravity 

greater   than   beta   and 

greater  than  alpha  iron 

smaller    than    gamma 

alpha   iron    (dilatation 

(gradual   dilatation   at 

and  beta  iron 

at  Ar3) 

Ar2,  Osmond) 

Electric  conductivity 

ten  times  smaller  than 

greater  than  that  of  gam- 

greater   than    that    of 

that  of  alpha  iron   at 

ma  iron  and  increasing 

beta  iron  and  increas- 

ordinary temperature 

with    falling    tempera- 

ing with  falling  tem- 

ture 

perature 

Magnetic  properties 

non-magnetic 

feebly  magnetic 

strongly  magnetic 

Hardness 

softer    than   beta   iron, 

very  hard 

soft 

harder  than  alpha  iron 

CHAPTER  XIII 

CAST  STEEL 

The  structural  and  other  changes  taking  place  at  the  thermal  critical  points  of 
steel  account  for  the  deep  changes  of  properties  resulting  from  the  treatments  to 
which  steel  is  subjected  in  the  process  of  manufacture  of  steel  objects.  We  are  now 
in  a  position  to  understand  these  changes,  to  anticipate  them,  and  to  arrive  at  the 
rationale  of  the  treatment  of  steel  which  for  so  many  centuries  remained  purely  em- 
pirical. 

It  is  logical  that  we  should  first  consider  the  structure  of  steel  before  it  has  re- 
ceived any  treatment  whatsoever,  namely,  the  structure  of  the  metal  in  its  cast  con- 
dition. To  this  study  the  present  chapter  will  be  devoted. 

The  structure  of  cast  steel  is  different  from  what,  in  these  chapters,  has  been 
termed  the  normal  structure  of  the  metal  because,  having  been  developed  during  very 
slow  and  undisturbed  cooling  from  the  molten  condition,  crystalline  growth  has  been 
promoted,  whereas  in  working  and  reheating  such  large  growth  is  hindered  or  cor- 
rected. It  may  well  be  expected  then  that  the  structure  of  steel  castings  will  be 
coarser,  as  is  generally  expressed,  that  is,  made  up  of  larger  crystalline  grains,  than 
the  normal  structure  so  far  considered,  and  therefore  that  steel  castings  will  suffer 
from  all  the  ills  that  pertain  to  a  coarse  structure,  namely,  weakness,  lack  of  ductility, 
or  even  brittleness,  etc.  It  will  be  profitable  to  consider  first  the  crystallization  of 
steel  in  general  and  then  the  genesis  of  the  structure  of  cast  eutectoid,  hypo-eutcc- 
toid,  and  hyper-eutectoid  steel. 

Crystallization  of  Steel.  —  We  are  indebted  to  Captain  Belaiew  more  than  to 
any  one  else  for  our  knowledge  of  the  crystallization  of  steel.  His  views  will  be  briefly 
described. 

The  crystallization  taking  place  during  the  solidification  period  is  called  by 
Belaiew  "primary  crystallization";  it  consists  in  the  formation  of  dendrites  of  a 
solid  solution  of  iron  and  carbon  (austenite),  all  steels  immediately  after  solidifica- 
tion being  built  up  of  juxtaposed  and  interlocked  dendrites  mutually  limiting  each 
other.  If  during  the  process  of  solidification  the  molten  mother-metal  is  withdrawn 
"isolated  crystals"  are  exposed  to  view  (see  Chapter  V,  Fig.  123).  The  dendrites  of 
the  primary  crystallization  are  composed  of  octahedra,  the  octahedron  being  the 
crystallographic  form  of  iron  and  steel.  According  to  Belaiew  this  primary  crystal- 
lization may  be  detected  on  the  surface  of  steel  samples  under  favorable  conditions 
while  it  may  be  brought  out  on  polished  surfaces  by  a  prolonged  etching  with  dilute 
acids  as  recommended  by  N.  J.  Belaiew.  The~structure  is  then  frequently  "mac- 
roscopic" that  is  visible  with  the  naked  eye.  A  remarkable  instance  of  this  primary 
dendritic  structure  in  manganese  steel  is  shown  in  Figure  190. 

Stead  depicts  this  primary  crystallization  in  Figure  191  which  represents  the 
gradual  growth  of  three  separate  crystals.  The  dark  crystallites  in  A  feed  and  cle- 

208 


CHAPTER   XIII  — CAST   STKKL 


209 


velop  out  of  liquid,  abstracting  the  iron  and  concentrating  the  carbon  in  portions 
that  still  remain  liquid.  The  white  junctions  in  ('  and  D  represent  the  last  portions 
of  liquid  metal,  rich  in  carbon,  also  sulphur  and  phosphorus,  if  any  happens  to  be 
present.  Stead  writes:  "The  fine  fir-tree  crystallites,  containing  probably  a  fraction 
of  the  amount  of  the  carbon  in  the  liquid  steel,  grow  steadily  forward  from  the  cold 
surfaces  of  the  containing  moulds.  The  crystallites  develop  branches  in  three  direc- 


Fig.  190.  —  Manganese  steel  magnified  til*  diameters. 

lions  corresponding  to  the  axes  of  the  cube,  and  these  branches  throw  out  similar 
branches  themselves.  Eventually  parts  of  the  most  fusible  portions  are  trapped 
between  the  branches  and  are  the  last  to  solidify.  When  there  is  much  phosphorus 
or  some  sulphur  in  the  metal,  they  are  always  present  together  with  an  excess  of  the 
i-arbon  in  the  la-t  residue  of  metal  that  remains  liquid  and  although  in  cooling  down. 
after  the  liquid  has  solidified,  the  excess  carbon  diffuses  out  of  it  into  the  purer  part. 


A. 


Fiji.   I'.H.  —  Diagram  representing  the  gradual  growth  of  three  separate  crystals.      (Stead.) 


the  sulphides  and  phosphides  do  not,  but  remain  fixed,  and  can  generally  be  detected 
in  the  solid  metal." 

Immediately  after  the  solidification  is  completed  a  crystalline  transformation 
starts  in,  which  Belaiew  calls  "granulation."  This  granulation  continues  until  the 
critical  range  of  the  metal  is  reached,  all  steels  being  then  made  up  of  a  number  of 
grains,  each  grain  built  up  of  small  octahedra  and  having  its  own  orientation.  This 
granulated  structure  generally  called  austenitic  or  polyhedral  (C.uillet)  may  be  re- 


210 


CHAPTER   XIII  — CAST   STEEL 


vealed  by  etching  the  steel  while  still  in  its  granulating  zone,  that  is  above  its  critical 
range,  with  hydrochloric  acid  (Osmond's  method) .  It  will  be  shown  elsewhere  that 
in  the  case  of  some  alloy  steels,  notably  with  certain  manganese  and  nickel  steels, 
this  granulated  structure  is  preserved  at  atmospheric  temperature  when  it  can  be 


$        *     . 

£  -v        1 

\  . 


Fig.  192.  —  Manganese  steel.  Cast.  Heated  to  1000  deg. 
C.  and  quenched  in  water.  Reheated  for  two  hours  at 
700  deg.  C.  and  furnace  cooled.  Magnified  100  diameters. 


Fig.  193.  —  Steel  containing  about  0.50  per  cent  carbon.     Cast.     Magnified  100 
diameters.     (R.  W.  Smyth  in  the  author's  laboratory.) 

revealed  by  the  ordinary  etching  methods  (Fig.  192).  Generally  speaking  the  longer 
the  sojourn  in  the  granulation  zone  and  the  slower  the  cooling  the  larger  will  be  the 
austenite  grains  on  reaching  the  critical  range  and,  as  later  explained,  the  coarser 
the  final  structure  of  the  steel  after  slow  cooling  through  the  range. 


CHAPTER  XIII  —  CAST  STEEL 


211 


In  passing  through  the  critical  range  as  previously  described  the  excess  ferrite  of 
hypo-eutectoid  steel  or  the  excess  cementite  of  hyper-eutectoid  steel  is  rejected  by 
each  austenite  grain  until  finally  in  cooling  through  An  the  remaining  portions  of 


Fig.  194.  —  Steel.      Hyper-eutectoid.      Magnified    100  diameters. 
(R.  W.  Smyth  in  the  author's  laboratory.) 


Fig.  195.  —  Hypo-eutectoid  steel.  Cast.  Free  ferrite 
rejected  chiefly  between  cleavage  planes.  Magni- 
fied 100  diameters.  (W.  J.  Burger,  Correspondence 
Course  student.) 


each  grain,  now  necessarily  of  eutectoid  composition,  are  converted  bodily  into  pearl- 
ite.  This  important  crystalline  transformation  taking  place  as  the  steel  cools  through 
its  critical  range  is  called  by  Belaiew  "secondary  crystallization."  The  resulting 


212 


CIIAI'TKI!    Mil       CAST    STKKL 


structures  which  are  retained  after  complete  cooling  are  readily  revealed  by  the  or- 
dinary etching  methods.  Very  slow  cooling  while  this  secondary  crystallization  is 
taking  place  promotes  the  rejection  of  the  excess  constituent  (ferrite  or  cement  it  e) 
to  the  boundaries  of  the  grains  giving  rise  after  complete  cooling  to  so-called  net- 
work or  cellular  structures  which  are  so  frequent  in  steel  forgings,  and  in  which  each 
grain  of  pearlite  is  surrounded  by  a  membrane  of  ferrite  in  hypo-eutectoid  steel  and 
of  cementite  in  hyper-eutectoid  steel  (Figs.  198  and  194).  If  during  the  second- 
ary crystallization,  however,  the  cooling  is  quite  rapid,  the  excess  constituent  is  for 


Fig.  190.  —  Stool.     Carbon  0.55  per  cent.     Widmanstatten  structure.     Magnified 

(i  diameters.      (  Helaiew. ' 


the  most  part  retained  between  the  cleavage  planes  of  the  small  octahedra  compos- 
ing each  grain,  probably  because  the  necessary  time  is  denied  for  this  excess  constit- 
uent to  reach  the  boundaries  of  the  grains.  This  type  of  structure  is  of  frequent 
occurrence  in  steel  castings  (Fig.  195)  and  is  intensified  by  a  long  sojourn  in  the  gran- 
ulation range  of  temperature  because  such  treatment  results  in  the  formation  of 
larger  grains.  This  structure  which  might  be  described  as  "cleavage"  structure  is 
frequently  called  "Widmanstatten"  structure  in  memory  of  A.  Widmanstatten  who 
in  1808  discovered  its  existence  in  certain  meteorites.  To  sum  up,  the  network  or 


CHAPTER    XIII  — CAST   STEEL 


2K5 


cellular  type  of  structure  is  developed  in  steel  castings  by  slow  cooling  through  the 
critical  range,  especially  through  the  upper  part  of  the  range  (i.e.  through  Ar3,  Ar2, 
Ar3.2.  or  Ar(.m,  as  the  case  may  be,  down  to  An)  when  the  excess  constituent  separates 
from  the  solid  solution.  The  longer  the  exposure  of  the  metal  in  the  granulation 
range,  that  is  between  the  end  of  the  solidification  and  the  beginning  of  the  critical 
range,  the  larger  generally  the  meshes  or  cells  and,  therefore,  also  the  pearlite  grains. 
Very  slow  cooling  through  the  granulation  range  also  promotes  the  formation  of  large 
grains  first  of  austenite  and  later  of  pearlite.  The  cleavage  or  Widmanstatten  type 
of  structure  is  promoted  by  long  sojourn  in  the  granulation  range  and  slow  cooling 
in  that  range  and  by  relatively  quick  cooling  through  the  critical  range,  especially 


Fig.  197.  —  Section  parallel  to  the  surface 
of  a  cube.      (Tschennak.) 


Fig.  U)8. — Steel.   Carbon  0. 55  per  cent.   Sect  ion  parallel 
to  the  surface  of  a  cube.     Magnified  30  diameters. 
(Belaiew.) 


through  the  upper  part  of  the  range  while  the  excess  constituent  is  being  rejected. 
The  Widmanstatten  structure  is,  generally  speaking,  more  brittle  than  the  network 
type. 

Belaiew  succeeded  in  a  remarkable  manner  in  reproducing  the  Widmanstatten 
structure  by  subjecting  carbon  steels  to  suitable  treatments.  The  structures  ob- 
tained by  him  in  the  case  of  steel  containing  0.55  per  cent  carbon  and  otherwise  of 
commercial  quality  are  shown  in  Figures  196  to  203.  They  are  typical  structures 
of  steel  castings  of  the  same  grade  but  on  a  much  larger  scale,  for  it  should  be  noted 
that  the  magnification  of  Figures  198  to  203  is  only  30  diameters  while  Figure  196 
is  magnified  but  6  diameters.  Figure  196  is  a  beautiful  illustration  of  that  type  of 
structure  in  which  the  free  ferrite  has  been  rejected  both  to  the  grain  boundaries 
forming  a  sharply  outlined  network  and  between  crystallographic  planes.  In  Figure 
198  to  203  the  free  ferrite  is  seen  massed  between  cleavage  planes. 


214 


CHAPTER   XIII  — CAST   STEEL 


Fig.  199.  —  Section  parallel  to  the  sur-    Fig.  200.  —  Steel.    Carbon  0.55  per  cent.    Section  parallel  to 
face  of  an  octahedron.    (Tsehermak.)          the  surface  of  an  octahedron.     Magnified  30  diameters. 

(Belaiew.) 


Fig.  201.  —  Section  parallel  to  the  sur-  Fig.  202.  —  Steel.  Carbon  0.55  per  cent.  Section  parallel 
face  of  a  dodecahedron.  Magnified  30  to  the  surface  of  a  dodecahedron.  Magnified  30  diame- 
diameters.  (Tsehermak.)  ters.  (Belaiew.) 


CHAPTER  XIII  — CAST   STEEL 


215 


Octahedric  Crystallization  of  Austenite.  —  It  will  be  noted  that  the  ferrite  bands  shown  in  Figures 
198  to  203  .;ut  each  other  at  right  angles  or,  more  frequently,  form  equilateral  triangles.  According 
to  crystallographers  these  are  indications  that  austenite  crystallizes  in  regular  octahedra.  That 
this  inference  is  correct  appears  to  be  conclusively  proven  by  the  following  remarks  of  Belaiew: 

"Let  us  consider  an  octahedron  and  let  us  assume  that  four  systems  of  lamellae  locate  themselves 
in  this  octahedron  along  its  crystallographic  planes,  that  is,  parallel  to  the  four  pairs  of  its  surfaces, 
a  fact  that  has  been  long  known  in  the  case  of  meteoric  irons. 

"  If  we  now  examine  any  section  of  the  octahadron,  we  shall  find  that  not  only  the  angles  formed 
by  the  projections  of  the  lamella?  vary  in  different  sections,  but  that  the  number  itself  of  different 
sections  varies  likewise  from  two  to  four.  For  instance,  when  the  section  is  parallel  to  the  surface 
of  the  cube,  the  number  of  different  sections  is  minimum,  that  is,  two,  and  in  the  entire  section  we 


Fig.  203.  —  Steel.    Carbon  0.55  per  cent.    Four  systems  of 
lamellae.     Magnified  30  diameters.     (Belaiew.) 


find  only  two  systems  of  lamella;  forming  right  angles.  Figure  197  is  a  diagram  of  such  section  and 
Figure  198  a  corresponding  section  of  the  steel. 

"A  section  parallel  to  one  of  the  surfaces  of  the  octahedron  will  yield  equilateral  triangles  formed 
by  three  systems  of  lamella)  forming  00°  angles;  the  fourth  system  coincides  with  the  section  con- 
sidered (see  Figs.  199  and  200). 

"In  a  section  parallel  to  the  surface  of  the  dodecahedron,  two  systems  of  lamellae  are  observed 
forming  an  angle  of  109°  28l  16'';  the  other  two  systems  coincide  and  divide  this  angle  in  half  (Figs.  201 
and  202).  Finally  any  section  will  give  four  different  systems  cutting  each  other  at  different  angles 
(Fig.  203). 

"All  these  cases,  as  we  have  just  seen,  can  very  well  be  illustrated  by  different  samples  of  our 
alloy  which,  firstly,  affords  a  rather  weighty  proof  of  the  octahedric  crystallization  of  steel  and,  sec- 
ondly, brings  out  the  remarkable  analogy  of  this  structure  with  that  of  meteorites  and  warrants  us 
to  allude  to  the  synthesis  of  the  structure  so  called  of  Widmanstatten  .  .  .  this  structure  is  the  neces- 
sary consequence  of  the  uniform  orientation  of  the  elementary  octahedra  within  a  volume  of  greater 
or  less  dimension ;  it  is,  therefore,  in  no  way  related  with  the  carbon  content  and  must  be  obtained 


216  CHAPTER   XIII  — CAST   STEEL 

in   any  alloy  of  iron  and  carbon  whenever  (lie  conditions  are  favorable  to  the  format  ion  of  that 
structure. 

"Moreover,  in  practice  this  structure  is  met  (although  certainly  much  less  developed  than  in 
our  alloys)  every  time  that  the  metal  is  subjected  to  an  intense  heating  followed  by  slow  cooling  as 
is  the  case  with  cast  steel  or,  better  still,  with  burnt  or  overheated  steel. 

"The  very  brittleness  of  these  steels  may  be  due  to  a  certain  extent  to  the  uniform  orientation 
of  the  elementary  octahedra  which  are  the  cause  of  that  structure." 

Let  it  be  noted  that  in  the  case  of  meteorites  the  length  of  time  during  which  the  metal  is  main- 
tained at  a  high  temperature  is  so  long  that  generally  but  one  crystal  is  formed,  that  is,  all  the  ele- 
mentary octahedra  formed  on  solidification  have  assumed  the  same  orientation.  In  steel  the  con- 
ditions being  less  favorable  to  uniformity  of  orientation  we  have  several  grains. 

Structure  of  Cast  Eutectoid  Steel.  —  Above  its  melting-point  eutectoid  steel 
consists,  like  all  steels,  of  a  liquid  solution  of  carbon  or  of  the  carbide  Fe3r  in  iron. 
It  solidifies  as  a  solid  solution  (austenite)  of  the  same  constituents,  dendritic  crystals 


Fig.  204.  —  Eutectoid  steel.     Cast.     Mag-    Eig.  20").  —  Hypo-euteetoid  steel.    Cast.    Free  ferrite 
nified  500  diameters.     (Boylston.)  rejected  chiefly  to  the  boundaries.     Magnified  100 

diameters.     (H.  C.  Cridland  in  the  author's  labora- 
tory. I 

made  up  of  small  octahedra  being  formed.  After  solidification  granulation  takes 
place  and  remains  active  down  to  the  beginning  of  the  critical  range  which  in  eutec- 
toid steel  is  reduced  to  the  single  point  Ar3.2.i,  the  metal  being  now  made  up  of  crystal- 
line grains  of  austenite.  In  passing  through  this  point  the  austenite  grains  are  con- 
verted bodily  into  as  many  pearlite  grains,  as  explained  in  Chapter  XI,  a  coarse 
austenitic  structure  acquired  at  a  high  temperature  giving  rise  to  a  coarse  pearlitie 
structure  at  ordinary  temperature.  The  polyhedral  .structure,  therefore  (Fig.  204), 
observed  after  complete  cooling  indicates  the  original  polyhedral  structure  of  austenite 
formed  above  the  critical  point.  Because  of  a  coarser  grain  and  coarser  pearlite  cast 
eutectoid  steel  is  weaker  and  less  ductile  than  eutectoid  steel  properly  worked  or 
annealed  or  both. 

Structure  of  Cast  Hypo-Eutectoid  Steel.  —  Let  us  now  consider  the  genesis  of  the 
structure  of  hypo-outectoid  steel,  and  let  us  select   as  an  example  steel   containing 


CHAPTER    XIII  — CAST   STKKL  217 

0.60  per  cent  carbon  and,  therefore,  composed  after  complete  slow  cooling  of  72  per 
cent  of  pearlite  and  28  per  cent  of  free  ferrite.  The  formation  of  the  structure  of 
this  steel  has  been  depicted  in  Figure  180,  Chapter  XI,  to  which  the  reader  is  referred. 
This  steel  on  solidifying  passes,  like  all  steels,  from  the  condition  of  a  liquid  solution 
of  iron  and  carbon  to  that  of  a  solid  solution  of  carbon  (or  more  probably  Fe3C)  in 
gamma  iron,  first  dendrites  and  then,  through  granulation,  crystalline  grains  of  aus- 
tcnite  being  formed.  These  grains  continue  to  grow  as  the  steel  cools  slowly  to  its 
upper  critical  point  Ar3.2  when,  as  explained  in  Chapter  XI,_ferrite  begins  to  be 
liberated  and  continues  to  be  liberated  as  the  metal  cools  to  its  lower  point  Ari.  This 
setting  free  of  ferrite  is  apparently  brought  about  by  each  grain  of  austenite  rejecting 
the  ferrite  in  excess  of  the  eutectoid  composition,  so  that  by  the  time  the  point  Ari 
is  reached  each  residual  grain  of  austenite  has  the  eutectoid  composition  and  on  cool- 
ing through  Ari  is  converted  bodily  into  a  grain  of  pearlite.  Microscopical  examina- 
tion reveals  the  fact  that  the  pro-eutectoid  ferrite  is  rejected  (1)  to  the  boundaries  of 
the  decreasing  austenitic  grains  and  (2)  between  the  cleavage  or  crystallographic 
planes  of  these  crystalline  grains,  so  that  three  types  of  structures  may  be  distin- 
guished in  cast  hypo-eutectoid  steel,  (a)  structures  in  which  the  free  (pro-eutectoid) 
ferrite  has  been  rejected  chiefly  to  the  boundaries  of  the  austenitic  grains  (Fig.  205), 
clearly  indicating  that  these  grains  were  polyhedral,  (b)  structures  in  which  the  free 
ferrite  has  been  rejected  chiefly  between  the  cleavage  planes  of  austenite  (Fig.  195), 
proving  the  crystalline  character  of  that  constituent  and  suggesting  that  its  crystal- 
lization is  cubic,  and  (c)  structures  in  which  the  free  ferrite  has  been  rejected  partly 
to  the  grain  boundaries  and  partly  between  the  cleavage  planes.  Long  exposure  to 
high  temperatures  followed  by  rapid  cooling  through  the  critical  range  appears  to 
favor  the  massing  of  free  ferrite  between  crystallographic  planes,  whereas  short  ex- 
posure and  slower  cooling  promotes  the  expulsion  of  free  ferrite  to  the  grain  bounda- 
ries, resulting  in  sharply  denned  network  structures. 

The  structure  of  cast  hypo-eutectoid  steel  is  coarse  (1)  because  its  slow  and  un- 
disturbed cooling  promotes  the  formation  of  large  austenite  grains  and  hence,  later, 
of  large  pearlite  grains,  (2)  because  its  slow  cooling  between  the  upper  and  lower 
critical  points  favors  the  rejection  of  a  maximum  amount  of  free  ferrite  which  rejec- 
tion makes  for  coarseness  of  structure,  and  (3)  because  its  slow  cooling  from  the 
upper  critical  point  to  atmospheric  temperature  promotes  the  crystallization  of  free 
ferrite  into  large  grains,  this  influence,  however,  being  material  only  where  there  is  a 
large  amount  of  free  ferrite,  i.e.  in  very  low  carbon  steel. 

Because  of  its  coarser  structure  cast  hypo-eutectoid  /steel  is  less  tenacious  and 
less  ductile  than  forged  or  properly  annealed  steel  of  similar  composition. 

Structure  of  Cast  Eutectoid  vs.  Structure  of  Cast  Hypo-Eutectoid  Steel. 
Although  the  pearlite  grains  of  eutectoid  steel  may  be  and  often  are  larger  than  the 
pearlite  grains  of  hypo-eutectoid  steel,  the  latter,  especially  when  judged  by  its  frac- 
ture, is  the  coarser  of  the  two.  This  greater  coarseness  of  hypo-eutectoid  steel  in 
spite  of  smaller  pearlite  grains  is  due  to  the  presence  of  free  ferrite,  relatively  small 
pearlite  grains  surrounded  by  coarse  ferrite  envelopes  or  holding  coarse  ferrite  par- 
ticles imparting  a  coarse  appearance  to  the  fracture  of  steel.  The  dimension  of  the 
pearlite  grains,  therefore,  while  not  without  influence,  is  not  the  criterion  by  which 
to  judge  of  the  coarseness  or  fineness  of  the  structure  and  fracture  of  hypo-eutectoid 
steel,  the  amount  of  free  ferrite  present  and  its  mode  of  distribution  having  to  be 
taken  into  consideration.  In  very  low  carbon  steel,  moreover,  there  is  but  little 


218  CHAPTER   XIII  — CAST   STEEL 

pearlite  and  the  small  amount  present  occurs  as  small  irregular  particles  exerting  but 
little  influence  upon  the  character  of  the  fracture  which  now  depends  quite  exclu- 
sively upon  the  dimension  of  the  ferrite  grains.  As  ferrite  grains,  however,  no  matter 
how  small  never  impart  as  fine  a  structure  or  fracture  to  steel  as  pearlite  grains,  it 
follows  that  low  carbon  (ferritic?)  steels  can  never  have  as  fine  a  structure  or  frac- 
ture as  higher  carbon  (pearlitic)  steels. 

Structure  of  Cast  Hyper-Eutectoid  Steel.  —  The  genesis  of  the  structure  of  cast 
hyper-eutectoid  steel  has  been  depicted  diagrammatically  in  Figure  181,  Chapter  XL 
Between  its  solidification  point  and  its  upper  critical  point  (Acn))  this  steel  is  com- 
posed, like  all  steels,  of  crystalline  austenite  grains.  Upon  reaching  the  point  Arcm 
the  setting  free  of  cementite  begins,  ending  only  at  the  lower  point  Ars.2.i-  This  free 


Fig.  206.  — •  Hyper-eutectoid  steel.  Cast.  Free  cementite  rejected 
partly  to  the  boundaries  and  partly  between  cleavage  planes. 
Magnified  114  diameters.  (Boylston.) 

cementite,  like  the  free  ferrite  of  hypo-eutectoid  steel,  is  rejected  (1)  to  the  bounda- 
ries of  the  diminishing  austenite  grains  and  (2)  between  the  cleavage  planes  of  this 
crystalline  austenite,  giving  rise  to  the  three  types  of  structure  described  in  the  case 
of  hypo-eutectoid  steel,  but  in  which  free  ferrite  is  replaced  by  free  cementite,  namely, 
(a)  structures  in  which  the  free  cementite  is  found  chiefly  at  the  grain  boundaries, 
(6)  structures  in  which  the  free  cementite  is  chiefly  located  between  cleavage  planes, 
and  (c)  structures  in  which  the  free  cementite  is  partly  at  the  boundaries  and  partly 
between  crystallographic  planes  (Fig.  206).  Like  the  structure  of  cast  hypo-eutec- 
toid steel,  the  structure  of  cast  hyper-eutectoid  steel  bears  witness  (1)  to  the  poly- 
hedric  form  of  the  austenite  grains,  (2)  to  the  crystalline  character  of  these  grains, 
and  (3)  to  their  probable  cubic  crystallization. 

Long  exposure  to  high  temperatures  followed  by  relatively  rapid  cooling  between 
Arcm  and  Ar3.2.i  promotes  in  hyper-eutectoid  steel  the  rejection  of  cementite  to  the 
cleavage  planes,  while  short  exposure  and  slower  cooling  favor  the  rejection  of  cemen- 
tite to  the  boundaries. 


CHAPTER  XIII  — CAST  STEEL 


219 


The  rejection  of  free  cementite,  like  the  rejection  of  free  ferrite,  makes  for  coarseness 
of  structure  and  fracture,  from  which  it  follows  that  cast  and  slowly  cooled  hyper- 
eutectoid  steel  will  be  coarser  than  cast  and  slowly  cooled  eutectoid  steel,  and  that 
the  more  free  cement  ite  it  contains,  that  is,  the  higher  the  carbon,  the  coarser  it  will 
be.  The  structure  and  fracture  of  hyper-eutectoid  steel,  however,  will  generally  be 
decidedly  finer  than  that  of  hypo-eutectoid  steel  because  of  the  very  small  amount  of 
free  cementite  present  in  the  former  compared  to  the  amount  of  free  ferrite  in  the 
latter,  unless,  indeed,  the  hypo-eutectoid  steel  be  very  near  the  eutectoid  composition. 
This  is  due  to  the  fact,  now  well  understood,  that  starting  from  the  eutectoid  com- 
position (carbon  0.85  per  cent),  as  the  carbon  decreases  the  amount  of  free  ferrite 
increases  rapidly,  while  as  the  carbon  increases  above  the  eutectoid  ratio  the  amount 


B 


D 


E 


H 


Hot 


„  Arj.i.     line 
H710°C. 


695° C.   i 


Cold 


CARBON  PEE  CENT 
0.12        0.20        0.50        O.Go        0.75        0.90         1.10         1.40         1.60 

Fig.  207.  —  (Stead.) 


of  free  cementite  increases  slowly  and  remains  small  even  with  high  carbon  content. 
Steel  with  0.50  per  cent  carbon,  for  instance,  contains  40  per  cent  of  free  ferrite,  hence 
its  coarseness  both  of  structure  and  fracture,  while  steel  with  say  1.25  per  cent  carbon 
contains  but  6.4  per  cent  of  coarsening  free  cementite,  hence  the  relative  fineness  of 
both  its  structure  and  fracture. 

The  rejection  of  the  "excess"  constituent  (ferrite  or  cementite)  and  the  formation  of 
the  final  pearlitic  structure  in  steels  of  varying  carbon  contents  has  been  interestingly 
depicted  by  Stead  (Fig.  207).  A,  B,  C,  D,  and  E  are  hypo-eutectoid  steels  containing 
increasing  percentages  of  carbon,  F  is  steel  of  eutectoid  composition,  G,  H,  and  I 
hyper-eutectoid  steels  of  increasing  carbon  contents.  The  diagram  clearly  shows 
(1)  that  in  hypo-eutectoid  steel,  as  the  carbon  increases,  the  rejection  of  the  free 
ferrite  (the  white  constituent)  by  the  solid  solution  (the  black  portion)  begins  at 
progressively  lower  temperatures  (the  Ara  or  Ars.2  point)  and  that  the  amount  re- 
jected decreases  steadily,  (2)  that  in  hyper-eutectoid  steels,  with  increasing  carbon, 
cementite  is  rejected  in  larger  proportions  and  at  increasing  temperatures  (the  Arcm 


220  CHAPTER  XIII  — CAST   STEEL 

point),  (3)  that  at  the  Ar^  or  Ars.s.i  point  the  remaining  solid  solution  (black  portion) 
is  converted  bodily  into  pearlite  (the  shaded  portion),  (4)  that  a  coarse  austenitic 
structure  developed  above  the  ATI  point  implies  the  formation  of  a  coar.se  pearlitic 
structure  below  that  point,  and  (5)  that  with  some  0.50  or  more  per  cent  carbon  the 
excess  constituent  frequently  forms  membranes  surrounding  pearlite  grains  and  giv- 
ing rise  to  so-called  network,  cellular,  or  polyhedral  structures. 

Ingotism.  —  Howe  has  suggested  the  term  "ingotism"  to  designate  the  structure 
of  cast  steel  described  in  the  foregoing  pages  and  characterized  (1)  by  large  pearlite 
grains  and  (2)  by  coarse  ferrite  or  cementite  membranes  surrounding  them  and  by 
irregular  masses  of  these  constituents  located  in  some  of  the  cleavage  planes  of  the 
original  austenite  grains. 


CHAPTER   XIV 

THE   .MECHANICAL   TREATMENT   OF   STEEL 

Forged  steel  objects  are  manufactured  by  subjecting  the  cast  metal  (1)  to  pres- 
sure exerted  by  rolls,  presses,  or  dies,  or  to  blows  from  hammers  and  (2)  by  reheating 
it  to  various  temperatures  for  various  lengths  of  time  and  cooling  it  at  various  rates. 
In  other  words  we  have  to  consider  (1)  the  mechanical  treatment  of  steel  and  (2)  its 
heat  or  thermal  treatment.  The  machining  of  steel  is  not  here  mentioned  since  it 
can  evidently  have  no  effect  upon  the  structure  and  properties  of  the  metal,  unless  it 
be  a  very  superficial  one. 

While  the  primary  purpose  of  working  steel  is  to  shape  it  into  useful  appliances 
and  while  the  primary  purpose  of  reheating  it  may  be,  and  often  is,  to  impart  to  the 
metal  such  plasticity  as  will  facilitate  its  being  so  shaped,  both  mechanical  and  heat 
treatments  deeply  affect  the  structure  of  steel  and  therefore  its  physical  properties. 
To  this  very  important  subject  the  next  four  chapters  will  be  devoted.  We  shall  first 
consider  the  influence  of  mechanical  treatment  and  then  that  of  heat  treatment. 

The  effect  of  work  upon  the  structure  and  properties  of  steel  greatly  depends 
upon  the  temperature  of  the  metal  while  it  is  being  worked,  the  expressions  "hot  work- 
in""  and  "cold  working"  being  common  ones,  the  former  meaning  working  the  steel 
while  hot,  and  the  second  working  it  while  cold,  more  specifically  at  atmospheric 
temperature.  In  these  pages  the  expression  hot  working  will  be  applied  to  working 
the  metal  while  above  its  critical  range,  and  cold  working  to  work  performed  below 
that  range.  In  justification  of  this  course  it  will  be  shown  that  the  effect  of  work 
changes  quite  sharply  as  the  critical  temperature  of  steel  is  passed. 

Hot  Working.  —  Hot  working  may  be  applied  to  steel  (1)  after  it  has  solidified 
but  before  it  has  cooled  to  a  much  lower  temperature  so  that  it  still  possesses  the 
necessary  plasticity,  or  (2)  the  steel  ingot  may  be  allowed  to  cool  to  atmospheric  tem- 
perature or  at  least  to  a  temperature  so  low  that  reheating  is  required  as  a  preliminary 
step  to  successful  hot  working.  The  following  considerations  will  show  that  so  far  as 
the  influence  of  hot  work  is  concerned  it  is  quite  immaterial  whether  the  steel  ingot 
has  or  has  not  been  completely  cooled  before  being  brought  to  the  forging  tempera- 
ture. Let  us  assume  that  the  steel  ingot  be  allowed  to  cool  to  atmospheric  tempera- 
ture, that  it  is  then  reheated  to  a  temperature  well  above  its  critical  range  and  then 
subjected  to  hot  working. 

An  attempt  has  been  made  in  Figure  208  to  depict  graphically  the  influence  of 
hot  work  on  the  structure  of  steel.  The  diagram  will  be  readily  understood.  The 
critical  range  both  on  heating  and  cooling  is  represented  by  a  double  line,  but  the 
reader  will,  of  course,  bear  in  mind  that  the  critical  range  on  heating  does  not  coincide 
with  the  range  on  cooling  and  that  each  range  may  include  one,  two,  or  three  critical 
points,  as  fully  explained  in  previous  chapters.  For  the  present  purpose  it  is  preferable 

221 


222 


CHAPTER  XIV  — THE   MECHANICAL   TREATMENT   OF   STEEL 


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4-  +  4-  +  +  +  +  +  4-  +  +  ' 


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H-  +  +  + 


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CHAPTER   XIV  — THE   MECHANICAL   TREATMENT   OF   STEEL  223 

to  represent  both  ranges  irrespective  of  the  number  of  critical  points  included  by  the 
two  parallel  lines  shown  in  the  diagram.  The  solidification  range  is  likewise  indi- 
cated by  a  double  line.  The  widths  of  the  shaded  areas  are  intended  to  be  propor- 
tional to  the  grain  size  resulting  from  the  various  treatments;  when  an  area  is  reduced 
to  a  mere  line  the  corresponding  grain  is  very  small,  i.e.  the  structure  very  fine.  As 
the  steel  ingot  solidifies  at  A  crystalline  grains  are  formed  which  increase  in  size  on 
slow  and  undisturbed  cooling  to  the  critical  range  B.  In  passing  through  that  range 
the  austenite  grains  are  converted  into  pearlitic  grains  with  or  without  rejection  of 
free  ferrite  or  of  free  cementite  according  to  the  carbon  content  of  the  steel.  On 
cooling  from  B  to  C  there  is  no  further  growth  and  the  size  of  the  final  grain  may  be 
represented  by  the  width  of  the  shaded  area  at  C.  The  metal  has  now  the  usual 
coarse  structure  of  steel  castings  described  in  Chapter  XIII.  Upon  reheating  this 
coarsely  crystalline  steel  ingot  from  S  to  R  and  through  its  critical  range,  it  is  con- 
verted from  the  condition  of  an  aggregate  of  ferrite  and  cementite  into  a  nearly  amor- 
phous solid  solution,  so  that  as  the  metal  emerges  from  its  range  it  has  a  very  fine 
structure.  As  steel  just  above  its  critical  range,  however,  would  not  be  plastic  enough 
for  hot  working  and  would  not  afford  the  necessary  range  of  temperature  through 
which  to  cool  while  being  worked  and  still  remain,  as  it  should,  above  the  range,  it 
is  generally  necessary  to  heat  the  metal  to  a  much  higher  temperature,  say  to  some 
1200  deg.  C.  or  even  more  (W  in  Fig.  208).  As  the  metal  is  heated  from  its  critical 
range  to  this  high  temperature,  it  probably  crystallizes  so  that  when  work  begins  at 
w  it  is  in  a  somewhat  crystalline  condition.  This  grain  growth  on  heating  from  R 
to  W  is  depicted  in  the  diagram.  The  heavy  pressure  or  blows  which  are  now  applied, 
however,  soon  break  up  this  crystallization  while  preventing  a  new  one  from  forming, 
so  long  at  least  as  the  work  continues  sufficiently  vigorous,  for  undisturbed  cooling 
is  a  condition  necessary  for  the  ready  growth  of  crystals.  As  soon  as  work  ceases, 
however,  the  metal  being  now  left  to  cool  undisturbedly,  if  its  temperature  is  then 
above  its  critical  range,  for  instance  at  /,  it  will  immediately  begin  to  crystallize 
and  its  crystallization  will  continue  from  /  to  r,  that  is  until  its  critical  range  is 
reached,  when  it  will  stop  and  the  austenite  grains  just  formed  will  be  converted  into 
as  many  pearlite  grains  with  or  without  rejection  of  free  ferrite  or  free  cementite. 
The  width  of  the  band  at  s  is  intended  to  represent  the  size  of  the  final  grain.  It  is 
smaller  than  the  grain  of  the  steel  ingot  but  it  is  still  large. 

If  work  ceases,  then,  while  the  metal  is  still  above  its  critical  range  it  is  evident 
that  it  will  crystallize  and  that  the  resulting  grains  will  be  the  larger,  that  is  its 
structure  will  be  the  coarser,  the  higher  the  temperature  at  which  the  work  was 
stopped. 

Finishing  Temperatures.  —  The  temperatures  at  which  work  ceases  are  known 
as  the  finishing  temperatures  and  the  above  considerations  will  show  the  importance 
of  proper  finishing  temperatures  if  it  be  desired  to  impart  to  steel  implements  the 
finest  grain  that  it  can  acquire  through  working  together  with  the  desirable  properties 
inherent  to  it. 

Should  hot  work  be  continued,  for  instance,  until  the  temperature  of  the  metal 
is  but  slightly  above  its  critical  range,  /'  in  Figure  208,  the  austenitic  grains  that  will 
form  on  undisturbed  cooling  to  the  range  will  be  small,  so  that  the  final  grain  of  the 
metal  at  s'  will  likewise  be  small,  as  indicated  by  the  width  of  the  shaded  area.  If 
the  finishing  temperature  is  exactly  at  the  critical  range,  /",  the  final  grain  at  s"  will 
be  very  fine.  If  working  be  continued  until  the  metal  has  cooled  to  a  temperature 


224 


CHAPTER   XIV  — THE    MECHANICAL   TREATMENT   OF   STEEL 


lower  than  its  critical  range,  for  instance  /'"  in  the  diagram,  the  structure  will  he 
fine  since  crystallization  was  prevented  while  the  metal  was  cooling  above  ils  critical 
range,  but  it  will  he  distorted  because  the  effect  of  working  below  the  range,  that  is 
of  cold  working,  is  to  distort  the  structure  as  explained  in  the  following  pages.  A 
distorted  structure  in  turn  means  decreased  ductility  and  eventually  hrittleness.  It 
seems  evident,  therefore,  that  hot  worked  steel  implements  should  be  finished  exactly 
at  their  critical  ranges  or  at  temperatures  but  slightly  superior  or  inferior  to  them. 

Structure  of  Hot  Worked  Eutectoid  Steel.  —  If  eutectoid  steel  be  subjected  to 
hot  work  until  it  has  nearly  reached  its  critical  range  (now  consisting  of  the  single 
point  Ar.vi.i),  /"  in  Figure  208,  it  will  have  a  very  fine  austenitic  structure  which  in 
cooling  slowly  and  undisturbedly  through  the  critical  range  at  r"  will  be  converted 


Fig.  209.  —  Steel.      Eutectoid.      Hot   workec 
Magnified  100  diameters.     (Boylston.) 


Fig.  210.  Hot  worked  hypo-eutectoid  steel. 
Carbon  ().">()  per  cent.  Finishing  temperature 
near  critical  range.  Magnified  100  diameter*. 
(Burger,  Correspondence  Course  student.) 


into  a  fine  pearlitic  structure.  The  metal  will  then  have  as  fine  a  structure  as  can 
l>e  imparted  to  it  by  work  alone,  followed  by  slow  cooling  through  the  range. 

If  eutectoid  steel  be  worked  until  its  temperature  is  still  considerably  above  its 
critical  point  (/,  Fig.  208),  and  then  allowed  to  cool  undisturbedly,  austenite  grains 
begin  to  form  and  increase  in  size  as  the  metal  cools  to  its  critical  point,  i.e.  from/  to 
/•  when  these  austenitic  grains  are  converted  into  as  many  pearlitic  grains.  The 
final  grain  size  therefore  will  depend  upon  the  temperature  at  which  work  ceased  and 
will  be  the  greater  the  higher  that  temperature. 

The  structure  of  hot  worked  eutectoid  steel  is  shown  in  Figure  209  under  a  mag- 
nification of  100  diameters. 

Structure  of  Hot  Worked  Hypo-Eutectoid  Steel.  —  If  hypo-euteetoid  steel  be 
worked  until  its  temperature  is  but  slightly  above  its  critical  range  (/",  Fig.  208) 
and  then  allowed  to  cool  undisturbedly,  small  grains  of  austenite  are  formed  which 
on  passing  through  the  critical  range  at  r"  are  converted  into  pearlite  grains  with 
rejection  of  excess  ferrite  as  now  well  understood  (Fig.  210).  When  finished  at 


CHAPTER   XIV  — THE    MECHANICAL   TREATMENT   OF   STEEL 


225 


higher  temperatures,  "network"  structures  are  often  produced  (Fig.  211)  the  bulk 
of  the  free  ferrite  forming-  membranes  surrounding  grains  of  pearlite  (or  of  sorbite). 
Generally  speaking  the  meshes  of  the  network  will  be  the  larger,  the  higher  the 
finishing  temperature.1 

In  the  case  of  low  carbon  steel,  however,  containing  say  less  than  0.30  per  cent 
carbon  the  proportion  of  free  ferrite  is  so  large,  i.e.  the  ferrite  net  so  thick,  that  the 
structure  consists  of  particles  of  pearlite  embedded  in  a  matrix  of  ferrite  (see  Fig. 
212  and  also  Fig.  207,  A  and  B). 

Structure  of  Hot  Worked  Hyper-Eutectoid  Steel.  —  If  hypeT-cntectoid  steel  be 
hot  worked  until  its  temperature  is  very  near  its  critical  range  (/",  Fig.  208)  and 
then  allowed  to  cool  undisturbedly,  small  austenite  grains  are  formed  which  on 
cooling  through  the  range  are  converted  into  small  pearlite  grains  with  rejection  of 


Fig.  211. — Hot    worked    hypo-eutectoid    steel.  Fig.  212.  —  Hot    worked    hypo-eutectoid    steel. 

Carbon  O.oO  percent.     Finishing  temperature  Carbon  0.05  percent.    Magnified  114  diame- 

considerably  above  the  critical  range.     Mag-  ters.     (Boylston.) 
nified  56  diameters. 


free  cementite,  as  shown  in  Figure  213.  If  the  work  be  stopped  at  a  temperature 
considerably  above  the  critical  range  (/,  Fig.  208),  the  final  pearlite  grains  will  be 
larger  while  the  free  cementite  will  be  located  chiefly  at  the  grain  boundaries,  a  net- 
work structure  being  produced.  It  will  be  evident  that  a  close  relation  exists  between 
the  size  of  the  meshes  of  these  network  structures  and  the  corresponding  finishing 
temperatures. 

Sorbite.  High  magnification  of  the  structure  of  the  meshes  of  both  hypo-  and 
hyper-cutectoid  steel  described  in  the  foregoing  pages  often  fails  to  reveal  the  char- 
acteristic features  of  pearlite,  namely,  (1)  sharply  defined  parallel  plates  alternately 

1  The  accuracy  of  this  statement  has  been  questioned  by  some  recent  writers,  notably  by 
W.  R.  Shinier,  who  studying  the  microstructure  of  steel  rails  finished  at  different  temperatures  con- 
cludes that  the  structure  is  affected  more  by  the  rate  of  cooling  than  by  the  finishing  temperature. 
The  author's  investigations,  on  the  contrary,  have  always  pointed  consistently  to  the  marked  in- 
fluence of  finishing  temperature  on  grain-size. 


226  CHAPTER    XIV  — THE    MECHANICAL   TREATMENT   OF   STEEL 

of  ferrite  and  cementite  and  (2)  a  constant  or  nearly  constant  carbon  content.  The 
structure  of  these  meshes  instead  remains  indistinct  and  presents  a  granular  rather 
than  a  lamellar  aspect  (Fig.  214).  It  is  also  frequently  noted  in  connection  with  these 
network  structures  that  the  full  amount  of  free  ferrite  or  of  free  cementite  has  not 
been  rejected,  the  pearlitic  grains  having  retained  some  of  the  constituent  in  excess  of 
the  eutectoid  ratio.  To  this  imperfectly  developed  pearlite  the  name  of  sorbite  has 
been  given  (Osmond)  and  quite  universally  adopted  in  spite  of  recent  and  regrettable 
efforts  to  eliminate  it  from  nietallographic  nomenclature.  It  will  be  apparent  that 
the  formation  of  sorbite  results  from  a  relatively  quick  cooling  through  the  critical 
range,  time  being  denied  for  the  crystallization  of  distinct  lamella;  of  ferrite  and 


Fig.  213.  —  Hot  worked  hypor-eutectoid  steel.  Carbon 
1.50  per  cent.  Finishing  temperature  near  critical 
range.  Magnified  100  diameters.  (Reinhardt  in  the 
author's  laboratory.) 

cementite  and,  in  the  cases  of  hypo-  and  hyper-eutectoid  stool,  for  the  rejection  of 
the  full  amount  of  free  ferrite  or  free  cementite.  The  cooling  in  air  of  hot  worked 
pieces,  especially  when  of  small  size,  is  often  sufficiently  rapid  to  cause  the  formation 
of  sorbite  rather  than  of  pearlite. 

The  production  and  nature  of  sorbite  will  be  dealt  with  at  greater  length  in  an- 
other chapter.  It  should  be  mentioned  here,  however,  that  while  sorbite  is  less  ductile 
than  pearlite  it  has  a  higher  tenacity,  higher  elastic  limit,  and  greater  hardness  (hence 
greater  wearing  power).  When  these  qualities  are  required,  in  hot  forged  objects, 
they  may  consequently  be  obtained,  although  necessarily  at  the  sacrifice  of  some 
ductility  and  softness,  by  hastening  the  cooling  through  the  critical  range,  after 
work  has  ceased,  when  sorbitic  rather  than  pearlitic  steel  will  be  produced. 

Hot  Working  of  Steel  vs.  Its  Critical  Range.  —  In  conducting  the  hot  working  of 
steel  so  as  to  impart  to  the  metal  the  finest  grain  that  can  result  from  finishing  at 
suitable  temperatures,  it  is  generally  necessary  only  to  consider  its  lower  critical 


CHAPTER   XIV --THE    MECHANICAL   TREATMENT   OF   STEEL 


227 


point  on  cooling,  namely,  Ar,  or  Ar3.=.i.  The  following  considerations  will  justify 
this  statement.  In  the  case  of  hypo-entertoid  steel  if  the  working  be  continued  while 
the  metal  cools  from  its  upper  point  or  points  to  its  lower  point,  it  will  make  for  fine- 
ness of  structure  by  preventing  a  coarse  massing  of  the  free  ferrite  while  the  cold 
working  of  that  constituent,  if  taking  place  at  all,  must  be  very  slight.  The  same 
reasoning  applies,  with  greater  force,  to  the  hot  working  of  hyper-eutectoid  steel  from 
its  upper  point  (denoting  the  formation  of  free  cementite)  to  its  lower  point  Ar3.2.i. 
(ireater  fineness  of  structure  will  result  with  very  little,  if  any,  distortion  of  the  free 
cementite. 

It  is  apparent,  therefore,  that  in  order  to  secure  the  finest  grain  obtainable  through 
mechanical  refining  without  appreciable  structural  distortion  steel  objects  should  be 


Fig.  214.  —  Hot  worked  sled.     Carbon  <)..->()  per  cent.     Magnified  650  diameters. 

finished  slightly  above  their  lower  critical  point,  that  is,  in  the  vicinity  of  700  deg.  C. 
for  all  grades  of  commercial  carbon  steel. 

Cold  Working.  —  By  the  cold  working  of  steel  is  meant  in  these  pages  the  work- 
ing of  it  while  its  temperature  is  below  its  critical  range.  .It  will  now  be  shown  that 
the  effect  of  cold  working  upon  the  properties  of  the  metal  is  very  different  from 
that  of  hot  working.  This  should  not  be  a  cause  for  surprise  if  it  be  borne  in  mind 
that  steel  above  its  critical  range  is  in  a  condition  totally  different  from  its  condition 
below  it.  Above  the  critical  range  we  have  to  deal  with  a  solid  solution  of  iron  and 
carl  .on,  below  it  with  an  aggregate  of  ferrite  and  cementite.  The  solid  solution  exist- 
ing above  the  range  will  crystallize  if  allowed  to  cool  undisturbedly  and  it  has  been 
shown  in  the  foregoing  pages  that  working  in  this  range,  i.e.  hot  working,  is  effective 
in  preventing  or  at  least  retarding  this  crystallization,  thus  imparting  a  smaller  grain 
to  the  metal.  The  aggregate  of  ferrite  and  cementite  existing  below  the  range,  on 


228  CHAPTER    XIV  — THE   MECHANICAL   TREATMENT   OF   STEEL 

the  contrary,  exhibits  no  tendency  to  crystallize  during  slow  and  undisturbed  cooling, 
because  this  .aggregate  was  formed  and  fully  developed  while  passing  through  the 
range,  the  size  of  its  elements,  that  is  its  coarseness,  depending  (1)  upon  the  coarse- 
ness of  the  solid  solution  immediately  before  its  transformation  and  (2)  upon  the 
time  occupied  in  cooling  through  the  range.  Working  this  aggregate,  therefore,  as  it 
cools  to  atmospheric  temperature,  or  working  it  while  at  atmospheric  temperature, 
i.e.  cold  working  steel,  does  not  prevent  its  crystallization.  Its  effect  consists  in  dis- 
torting the  existing  aggregate  structure,  chiefly  through  the  stretching  or  elongation 
of  its  crystalline  elements  (free  ferrite,  free  cementite,  pearlite)  in  the  direction  of  the 
forging,  and  such  distortion  in  turn  means  decreased  ductility  and  eventually  extreme 
brittleness.  The  effect  of  cold  working  upon  the  structure  of  steel  is  illustrated  in 


Eig.  215.  —  Cold  worked  hypo-cut eel oid  steel.  Car- 
bon 0.30  per  cent.  Magnified  100  diameters. 
(Burger,  Correspondence  Course  student.) 

Figures  215  and  216  in  the  case  of  hypo-eutectoid  steel.  It  is  also  depicted  in  the 
diagram  of  Figure  208.  While  the  structural  distortion  caused  by  cold  working  is  very 
slight  near  the  critical  range  of  the  metal,  it  rapidly  increases  as  the  temperature 
decreases,  becoming  very  pronounced  at  atmospheric  temperature.  The  manufacture 
of  wire  by  cold  drawing  affords  a  familiar  instance  of  the  effect  of  work  performed  at 
atmospheric  temperature  both  on  the  structure  and  properties  of  the  metal.  It  is 
well  known  that  after  the  wire  has  been  passed  through  several  dies  it  becomes  so 
brittle  that  annealing  is  necessary  in  order  to  make  further  reduction  in  size  possible, 
the  annealing  operation  removing  the  structural  distortion  and  brittleness  produced 
by  working  at  atmospheric  temperature. 

The  cold  working  of  iron  and  steel  affects  deeply  many  of  the  properties  of  these 
metals,  its  action  increasing  in  intensity  as  the  temperature  falls,  being  therefore  more 
severe  at  atmospheric  temperature.  The  elastic  limit,  tensile  strength,  and  hardness 
are  increased  in  a  marked  degree,  while  the  ductility  as  represented  both  by  elongation 
and  reduction  of  area  is  reduced,  extreme  brittleness  being  eventually  produced. 
1 1  is  generally  believed  that  cold  working  decreases  the  density  of  iron  and  steel  and 


CHAPTER   XIV  — THE   MECHANICAL   TREATMENT   OF   STEEL 


229 


that  the  annealing  of  the  cold  worked  metals  increases  their  density.  Both  magnetic 
permeability  and  remanence  are  diminished  by  cold  working  while  the  coercive 
force  is  increased.  Electrical  resistance  is  slightly  increased  as  well  as  the  solubility 
of  the  inelals  in  acids.  The  specific  heat  does  not  appear  to  be  affected. 

It  has  been  explained  in  Chapter  V  that  the  straining  of  metals  does  not  involve 
an  actual  distortion,  an  elongation  for  instance,  of  their  crystalline  elements,  but 
rather  a  yielding  through  successive  crystalline  slips.  It  should  also  be  recalled 
that  many  believe  in  the  existence  of  amorphous  iron  produced  by^seyere  straining,  as 
in  cold  working,  and  that  the  influence  of  that  operation  on  some  of  the  properties  of 


_« *^ — — •*•*  v*T~H»  T^"^"    .    ' 

^L^ife^^r^--*  ^-^ 

«  -^^ay-^^. 


5^l»r«B"^ 

-^v«*,  -  .,  ^>i*-v-«^ 


ifT-  216.  —  Cold  worked  hypo-eutertoid  steel.     Carbon  0.30  per  cent. 
Magnified  loO  diameters.     (Buck,  Correspondence  Course  student.) 


the  metal,  as  mentioned  above,  may  be  accounted  for  in  accordance  with  that 
hypothesis. 

Mechanical  Refining.  —  It  would  seem  as  if  with  the  use  of  pyrometers  at  least 
it  should  be  a  relatively  simple  matter  to  finish  steel  objects  very  near  the  desirable 
temperature  and  thus  secure  for  them  the  best  structure  that  can  be  imparted  by 
work  alone.  Upon  reflection,  however,  it  will  be  manifest  that  the  problem  is  on  the 
contrary  an  insoluble  one,  for  the  reason  that  unless  the  objects  are  of  very  small 
cross-sections,  it  is  quite  impossible  to  finish  them  so  that  their  temperature  will  be 
uniform  throughout,  the  central  portions  being  necessarily  hotter  than  the  outside. 
Should  the  forging  be  so  conducted  that  the  temperature  of  the  outside  be  very  near 
the  critical  range,  the  center,  being  materially  hotter,  will  coarsen  on  cooling,  while  if 
the  implements,  on  the  contrary,  are  finished  so  that  their  center  may  have  the  fine 
structure  produced  by  ceasing  the  work  at  the  proper  temperature,  their  outside  must 
necessarily  suffer  from  cold  working.  The  limitations  of  work  alone  as  a  means  of 
imparting  the  best  possible  structure  to  steel  are  therefore  quite  evident. 


CHAPTER   XIV  — THE    MECHANICAL   TREATMENT   OF   STEEL 

While  a,  uniformly  fine  grain  cannot  be  imparted  to  steel  objects  of  considerable 
size  through  hot  working  alone,  the  value  of  hot  work  as  a  means  of  refining  the 
structure  of  steel  remains  very  great  as  exemplified  by  the  structure  of  properly  hot 
forged  steel  when  compared  with  the  structure  of  steel  castings  of  similar  composi- 
tion. The  finer  grain  imparted  to  steel  by  working  it  has  been  called  by  some  writers 
"mechanical"  refining  to  distinguish  it  from  the  refining  produced  by  heat,  i.e.  from 
"thermal"  refining.  In  practise  hot  work  should  be  so  conducted,  that  is,  the  finish- 
ing temperatures  so  regulated,  that  the  central  portions  of  the  finished  implements 
will  not  suffer  unduly  from  the  coarsening  influence  of  too  high  a  finishing  tempera- 
ture, while  at  the  same  time  the  outside  will  not  suffer  unduly  from  the  effect  of  cold 
working.  The  natural  tendency  of  rolling  and  other  forging  mills  is  to  finish  work 
at  too  high  temperatures  for  the  simple  reason  that  the  metal  is  then  more  plastic 
and  consequently  requires  less  power  for  its  working.  In  some  manufactures,  how- 
ever and,  especially  in  the  rolling  of  rails,  the  importance  of  proper  finishing  tempera- 
tures has  been  given  careful  attention  and  the  rolling  operation  so  modified  as  to 
deliver  rails  of  much  finer  grain  and  therefore  better  physical  quality,  than  formerly. 

Besides  its  important  grain  refining  influence  hot  work  further  improves  the  quality 
of  steel  by  closing  and,  if  the  carbon  be  low  enough,  welding,  the  blow-holes  and 
otherwise  increasing  its  soundness  and  by  removing  cooling  strains. 


CHAPTER  XV 

THE   ANNEALINC    OF   STEEL 

Purpose  of  Annealing.  —  The  purpose  of  annealing  steel  may  be  (1)  to  increase 
its  softness  and  ductility  that  it  may,  for  instance,  be  more  easily  machined  or  (2)  to 
secure  a  desirable  combination  of  high  strength  and  elastic  limit  with  fair  ductility 
that  it  may  successfully  stand  the  strains  to  which  it  is  to  be  subjected.  These  changes 
of  physical  properties  result  from  corresponding  changes  in  the  structure  of  the  metal 
brought  about  by  proper  heat  treatment.  In  annealing  steel  it  is  generally  intended 
to  impart  to  it  as  fine  a  structure,  that  is  as  small  a  grain,  as  is  consistent  with  the 
nature  of  the  treatment  and  the  grade  of  the  steel.  Hot  forged  steel  objects  may  be 
improved  by  annealing  for  certain  purposes,  because  of  their  structure  being  often 
(1)  relatively  coarse  owing  to  high  finishing  temperature  and  (2)  heterogeneous  as 
explained  in  Chapter  XIV. 

Cold  worked  steel  must  generally  be  annealed  in  order  to  increase  its  ductility. 
A  large  amount  of  cold  drawn  wire,  however,  is  used  in  its  cold-worked  condition  when 
very  high  yield  point  and  tensile  strength  are  desired  and  relatively  low  ductility 
permissible,  as  in  spring  wires,  piano  (music)  wire,  wires  for  wire  ropes,  etc.,  when  a 
tensile  strength  exceeding  350,000  Ibs.  per  sq.  in.  may  be  obtained.1  Finally,  steel 
cast  ings  have  so  coarse  a  structure  as  to  be  very  deficient  both  in  strength  and  ductil- 
ity and  should  always  be  refined  by  annealing. 

Nature  of  the  Annealing  Operation.  —  The  annealing  operation  comprises  three 
distinct  steps:  (1)  heating  the  steel,  (2)  keeping  its  temperature  constant  at  the  an- 
nealing temperature,  and  (3)  cooling  it  from  the  annealing  to  atmospheric  tempera- 
ture. These  steps  will  be  considered  separately. 

Heating  for  Annealing.  —  The  first  step  in  annealing  always  consists  in  heating 
the  metal  past  its  critical  range  because  by  so  doing  the  preexisting  structure,  how- 
ever coarse,  is  obliterated;  the  metal,  for  the  time  being,  assuming  a  nearly  amorphous 
structure.  It  is  true  that,  as  later  explained,  cold  drawn  wire  is  very  frequently  re- 
heated to  a  temperature  inferior  to  its  critical  range  for  the  purpose  of  removing  the 
loss  of  ductility  occasioned  by  cold  working  but  it  may  well  be  contended  that  such 
an  operation  is  not,  strictly  speaking,  annealing,  for  it  does  not  imply  a  complete 
obliteration  of  the  preexisting  structure  such  as  takes  place  when  steel  is  heated  past 
its  critical  range. 

This  important  structural  change  is  due,  as  we  now  understand  it,  to  the  pas- 
sage of  the  steel  from  the  state  of  an  aggregate  of  ferrite  and  cementite  to  that  of 
a  homogeneous  solid  solution,  and  it  is  not  to  be  wondered  at  that  so  radical  a  struc- 
tural change  should  destroy  effectively  any  preexisting  crystallization.  The  anneal- 
ing of  steel  castings,  however,  constitutes  an  apparent  exception  to  the  rule  that 

1  The  author's  attention  was  called  to  these  important  uses  of  cold  worked  wire  in  the  un- 
aime;ded  condition  by  Mr.  K.  H.  IVirpe  of  the  American  Steel  and  Wire  Co. 

231 


232 


CHAPTER   XV  — THE   ANNEALING   OF   STEEL 


heating  just  through  the  range  is  sufficient  to  break  up  effectively  the  preexisting 
structure,  for  their  successful  annealing  often  requires  a  materially  higher  tempera- 
ture. Should  the  temperature  of  the  steel  remain  below  its  critical  range,  no  struc- 
tural change  would  take  place  and  the  annealing  would  be  ineffective  (I,  Fig.  217) 
unless,  indeed,  in  the  case  of  cold  worked  hypo-eutectoid  steel  when  the  distorted  fer- 
rite  may  be  restored  to  its  normal  crystalline  structure  by  heating  slightly  below  the 
critical  range  as  later  explained.1  Should,  on  the  contrary,  the  temperature  of  the 
steel  be  carried  considerably  above  the  range,  its  structure,  which  was  finest  as  it 
emerged  from  the  range,  begins  to  coarsen  on  further  heating  and  continues  to  grow 
as  the  metal  cools  slowly  to  the  range,  so  that  its  final  structure  would  be  at  least 


27 


Ar 


m 


J_ 


I  Cnf/caf 


3fruc  fure 


Coarse 
structure 


O 
f~/ne 


A 

structure 


F 


Very   f/ne    structure 
Strong,  etosf/c,   one"  /ot/p/j 

O     Hardest,  sfronues/,  uno*  /east  duc//7e 
duct/fa 


Fig.  217.  —  Diagram  dnpirtintj  1lu>  annealing  of  steel. 


relatively  coarse  (II,  Fig.  217).  Clearly,  therefore,  to  anneal  .steel  forgings  they 
should  be  heated  through  their  critical  range  and  kept  at  a  temperature  as  close  to 
the  upper  part  of  that  range  as  possible  (III,  Fig.  217).  The  annealing  temperature 
will,  of  course,  vary  with  the  carbon  content  since  the  position  of  the  critical  range, 
or  rather  its  width,  varies  likewise.  The  structural  refining  taking  place  in  heating 
through  the  critical  range  is  strikingly  illustrated  by  Stead  in  Figure  218,  which 
shows  coarsely  crystalline  soft  steel  heated  at  one  end  to  above  900  deg.  C.  The  re- 
fined portion  was  heated  above  the  critical  range.  The  sharp  demarcation  between 
the  original  coarse  structure  and  the  fine  structure  produced  by  annealing  should  be 
noted. 

1  When  steel  contains  hardening  carbon  it  may  lie  softened  and  made  more  ductile  by  heating 
it  to  temperatures  lower  than  its  critical  range  (as  in  the  tempering  of  hardened  steel)  but  such  treat- 
ment is  not,  or  at  least  should  not  be,  called  annealing.  Cooling  strains  may  also  be  removed,  at 
least  in  part,  by  heating  below  the  range. 


CHAPTER  XV  — THE  AXXEALIXG  OF  STEEL 


233 


The  following  ranges  of  temperatures  are  recommended  by  the  Committee  on 
Heat  Treatment  of  the  American  Society  for  Testing  Materials.  The  report  of  the 
Committee  states  that  for  steels  containing  more  than  0.75  per  cent  manganese 
slightly  lower  temperatures  suffice. 


RANGE  OF  CAKBON  CONTENT 

Less  than  0.12  per  cent 
0.12  to  0.2.5  per  cent 
0.30  to  0.49  per  cent 
0.50  to  1.00  per  cent 


RANGE  or  ANNEALING  TEMPERATURE 

S75  to  925  deg.  C.  (1607-1697  deg.  F.) 
840  to  870  deg.  C.  (1544-1598  deg.  F.) 
815  to  840  deg.  C.  {1499^1554  deg.  F. ) 
790  to  815  deg.  C.  (1454-1499  deg.  F.) 


The  proper  temperatures  to  which  to  heat  carbon  steels  of  ordinary  commercial 
quality  for  the  purpose  of  annealing  as  well  as  of  hardening  are  indicated  graphic- 
ally in  Figure  219  for  various  percentages  of  carbon. 


Fig.  218.  —  Coarsely  crystalline  soft  steel  heated  at   one  end  to  above 
900  deg.  C.     (Stead.) 


Time  at  Annealing  Temperature.  —  The  steel  object  should  be  kept  at  the  anneal- 
ing temperature  long  enough  to  be  heated  right  through  to  that  temperature.  The 
( lommittee  on  Heat  Treatment,  referred  to  above,  states  that  an  exposure  of  one  hour 
should  be  long  enough  for  pieces  twelve  inches  thick.  Thicker  pieces,  of  course,  need 
a  longer  heating. 

The  usefulness  of  pyrometers  in  conducting  annealing  operations  is  obvious. 
Their  use  is  to  be  strongly  recommended. 

Cooling  from  Annealing  Temperature.  —  Having  imparted  a  fine  structure  to  the 
steel  the  next  step  must  be  to  retain  it.  The  most  effective  way  of  accomplishing 
this  consists  in  cooling  the  steel  very  quickly,  by  quenching  it  in  water  for  instance, 
as  time  is  then  denied  for  the  structure  to  coarsen  at  all  while  the  metal  cools  to  at- 
mospheric temperature.  Such  rapid  cooling,  however,  as  is  well  known,  hardens  the 
metal  and  deprives  it  of  ductility  (unless,  indeed,  it  contains  very  little  carbon),  and 
this  would  defeat  the  purpose  of  annealing  which  always  demands  the  retention  of 
considerable  ductility.  It  follows  from  these  considerations  that,  in  annealing,  cool- 
ing from  the  annealing  temperature  cannot  be  so  rapid  as  to  very  materially  harden 
the  steel.1  Its  rate  should,  moreover,  be  regulated  in  accordance  with  the  kind  of 

1   1'nless  (lie  double  treatment  presently  to  be  described  is  resorted  to. 


234 


CHAPTER    XV  —  THK   ANNEALING    OF   STEEL 


properties  we  most  desire  the  steel  object  to  possess.  For  instance,  (1)  if  softness 
and  ductility  are  wanted  (for  ease  in  machining),  necessarily  at  a  certain  sacrifice  of 
strength  and  elasticity,  the  cooling  should  be  very  slow,  to  wit,  with  the  furnace  in 
which  the  object  was  heated,  (2)  if  greater  hardness  (for  wearing  power),  strength, 
and  elasticity  are  desired,  at  the  necessary  sacrifice  of  some  ductility,  the  cooling 
should  be  more  rapid  as,  for  example,  in  air  or,  in  the  case  of  low  carbon  steel,  in  oil 
or,  with  very  low  carbon  steel,  even  in  water  (III,  Fig.  217). 

Rate  of  Cooling  vs.  Carbon  Content.  —  The  lower  the  carbon  content  the  more 
rapid  may  be  the  cooling  from  the  annealing  temperature  without  affecting  too  deeply 


950 
40 
3O 
20 
10 

<300 

90 
80 
70 
60 
850 
4O 
30 

to 

10 

800 


1742T 


6  -7 

CARBON 


-8  -9  1O 

PER  CENT 


1.2 


Fig.  219.  —  •  Diagram  showing  suitable  temperatures  for  annealing  (and   hardening)   carbon  steel 

forgings. 


the  ductility  of  the  metal.  For  instance,  (1)  steel  containing  not  over  0.15  per  cent 
carbon  may  be  quenched  in  water,  therein"  increasing  its  strength  and  elastic  limit 
and  still  remain  very  ductile,  (2)  steel  with  less  than  0.20  or  0.30  per  cent  carbon  may 
be  quenched  in  oil  with  satisfactory  results,  (3)  with  a  larger  proportion  of  carbon 
such  rapid  cooling  is  no  longer  possible,  as  it  would  destroy  the  ductility  of  the  metal, 
recourse  having  then  to  be  had  to  cooling  in  air  for  the  desired  combination  of  strength 
and  ductility  or  to  the  double  annealing  treatment  soon  to  be  described. 

Rate  of  Cooling  vs.  Size  of  Object.  ---  Since  large  objects  necessarily  cool  more 
slowly  than  smaller  ones  when  subjected  to  the  same  cooling  influences,  it  is  evident 
that  the  external  conditions  should  also  be  regulated  in  accordance  with  the  dimen- 
sions of  the  objects  treated.  To  secure  maximum  softness  and  ductility,  for  instance, 
the  cooling  of  small  objects  should  be  more  effectively  retarded  than  the  cooling  of 


CHAPTER.  XV  — THE   ANNEALING  OF  STEEL 


235 


larger  ones.  Assume,  for  example,  two  objects  made  of  the  same  steel,  one  large  and 
one  small,  and  both  cooled  in  air  from  the  annealing  temperature;  the  smaller  object 
will  be  harder  and  less  ductile  than  the  larger  one,  because  of  its  quicker  cooling.  To 
render  it  as  soft  and  ductile  as  the  larger  object  cooling  in  the  furnace  may  be  neces- 
sary. Similarly,  to  give  strength  and  high  elastic  limit  the  cooling  of  large  objects 
must  be  more  vigorously  hastened  than  that  of  smaller  objects  as,  for  instance,  cool- 
ing in  oil  against  cooling  in  air  for  the  smaller  piece. 

Furnace  Cooling  from  Annealing  Temperature.  —  As  an  example  of  the  effect  of 
furnace  cooling  upon  the  structure  of  steel,  let  us  take  a  steel  bar  }^  inch  square,  con- 
taining 0.50  per  cent  carbon,  heated  to  1000  deg.  C.  and  slowly  cooled  with  the  fur- 
nace. Its  structure  is  shown  in  Figures  220  and  221.  It  will  be  seen  to  be  composed 


Fig.  220.  —  Steel.  Carbon  0.50  per  cent.  Magni- 
fied 100  diameters.  Heated  to  1000  deg.  C. 
and  slowly  cooled  in  furnace.  (R.  W.  Smyth 
in  the  author's  laboratory. ! 


of  the  normal  proportions  of  pearlite  and  free  ferrite,  namely,  some  60  per  cent  of  the 
former,  and  it  will  also  be  noted  that  the  pearlite  is  distinctly  laminated  (Fig.  221), 
and  that  in  places  at  least  the  ferrite  forms  characteristic  polyhedral  grains.  This 
structure  is  due  to  the  slow  cooling  of  the  steel  through  its  critical  range,  which  per- 
mits the  rejection  of  the  full  amount  of  free  ferrite  and  a  distinct  crystallization  of 
the  constituents  of  the  residual  austenite  into  plates  of  ferrite  and  cementite.  The 
relative  softness  and  great  ductility  of  the  steel  in  this  condition  is  due  (1)  to  the 
presence  of  the  full  amount  of  soft  ferrite  in  relatively  large  areas  and  (2)  to  the  pres- 
ence of  distinctly  laminated  pearlite  indicating  the  absence  of  hardening  carbon  as 
explained  later. 

Air  Cooling  from  Annealing  Temperature.  —  To  illustrate  the  influence  of  air 
cooling  upon  the  structure  of  steel,  let  us  take  likewise  a  steel  containing  some  0.50 
per  cent  carbon,  heated  to  1000  deg.  C.  and  cooled  in  air.  Its  structure  is  shown  in 


236 


CHAPTER   XV  — THE   ANNEALING    OF   STEEL 


Figure  222.  It  will  be  found  quite  unlike  the  structure  of  the  same  steel  after  fur- 
nace cooling  (Figs.  220  and  221).  It  contains  a  much  smaller  proportion  of  free  fer- 
rite,  apparently  not  over  20  per  cent,  in  the  form  of  a  distinct  net  surrounding  dark 
meshes  which  a  high  magnification  fails  to  resolve  into  distinct  parallel  plates.  Rela- 
tively quick  cooling  through  the  critical  range  has  prevented  the  separation  of  the 
normal  amount  of  free  ferrite,  from  which  it  necessarily  follows  that  the  dark  con- 
stituent contains  more  ferrite  than  true  pearlite;  nor  has  it  the  structure  of  true 
pearlite,  time  also  having  been  denied  on  cooling  through  the  range  for  the  forma- 
tion of  distinct  plates  of  ferrite  and  cementite.  Sorbite  is  the  name  of  this  constituent. 
The  structure  of  pearlite  passing  into  sorbite  is  shown  in  Figure  223. 

Properties  of  Sorbite.  —  Sorbite  has  already  been  briefly  described  in  Chapter 
XIV,  where  it  was  shown  that  it  could  be  produced  in  steel  forgings  of  small  sections 


Fig.  221.  —  Steel.  Carbon  0.50  per  cent.  Magnified  670  diameters.  Heated 
to  1000  deg.  C.  and  slowly  cooled  in  furnace.  (C.  C.  Buck,  Correspondence 
Course  student.) 


through  simple  air  cooling  from  a  finishing  temperature  superior  to  the-critical  range, 
and  in  larger  sections  by  hastening  somewhat  their  cooling  through  that  range.  It 
has  also  been  stated  that  sorbite  is  harder,  stronger,  and  less  ductile  than  pearlite. 
By  so  regulating  the  cooling  from  the  annealing  temperature,  therefore,  that  sorbitic 
steel  is  produced,  hardness,  strength,  and  elasticity  will  be  promoted  at  the  sacrifice 
of  some  ductility  (III,  Fig.  217).  It  will  be  explained  in  another  chapter  that  sorbite 
is  generally  regarded  as  one  of  the  transition  stages  assumed  by  the  metal  as  it  passes 
from  its  austenitic  condition,  stable  above  the  critical  range,  to  its  pearlitic  condition, 
stable  below  that  range. 

Influence  of  Maximum  Temperature.  —  The  influence  of  the  maximum  tem- 
perature to  which  steel  is  heated  before  being  allowed  to  cool  is  well  shown  in  Figures 
224  to  227  which  should  be  compared  with  Figures  220  to  223.  They  refer  to  steel 


CHAPTER   XV  — THK   AXXKAIJXG   OF   STEEL 


237 


Fig.  222.  —  Steel.  Carbon  0.50  per  cent.  Magnified  100 
diameters.  Heated  to  1000  deg.  C.  and  cooled  in 
air.  (Boynton  in  the  author's  laboratory.) 


Fig.  223. —  Steel.     Carbon   1.00  per  rent.      Magnified   1500  diameters. 
Pearlite  (laminated )  passing  into  sorbite.     (Osmond.) 


Fig. 224.  — Magnified  100  diameters.    Healed  to      Fig.. 225.  — Magnified  100 diameters.   Healed  to 
800  deg.  C.  and  slowlv  cooled  in  furnace.  SOO  deg.  C.  and  eooled  in  air. 


Fig.  226.  —  Magnified  070  diameters.     Healed  to          Fig.  227.  —  Magnified  070  diameters.     Healed 
800  deg.  C.  and  slowly  cooled  in  furnace.  to  SOO  deg.  C.  and  cooled  in  air. 

Figs.  224-227.  —  Steel.     ( !arbon  0.50  per  cent.     (C.  C.  Buck,  Correspondence  Course  student.) 

238 


CII.U'TKK    XV  — THK    ANNKAI, IXC    OF    STKKL  239 

containing  0.50  per  cent  carlion  and  heated  to  800  deg.  (.'.,  while  the  structures 
illustrated  in  Figures  220  to  223  refer  to  the  same  steel  but  heated  to  1000  deg.  The 
constituents  are  the  same,  namely,  ferrite  and  pearlite  in  the  furnace  cooled  samples, 
ferrite  and  sorbitc  in  the  air  cooled  samples,  but  the  higher  temperature  resulted  in 
the  formation  of  larger  particles  of  pearlite  or  sorbite,  evidently  because  of  the  forma- 
tion above  the  critical  range  of  larger  austenitie  grains. 

Influence  of  Time  at  Maximum  Temperature.  —  Maintaining  steel  for  a  long 
time  at  a  high  temperature  causes  the  formation  of  large  austenite  grains,  which  in 
passing  through  the  range  are  converted  into  large  pearlite  or~Rorbite  grains  with 


Fifi.  22S.  -Steel.  Carbon  O.oO  prr  cent.  Magnified  100  diameters. 
Heated  to  11  ">0  deg.  C.  for  two  hours  and  cooled  in  air.  (Hoynton 
in  (lie  author's  laboratory.) 

rejection  of  free  ferrite  in  hypo-eutectoid  steel  and  of  free  cenientitc  in  hyper-eutec- 
toid  steel.  It  is  also  noted  that  the  more  prolonged  the  heating  the  smaller  the 
amount  of  the  excess  constituent  (free  ferrite  or  free  cementite)  separating  on  rapid 
(air)  cooling  through  the  range.  This  is  shown  in  Figure  228  in  which  is  depicted  the 
structure  of  steel  containing  0.50  per  cent  carbon  heated  to  1150  deg.  C1.  for  two 
hours  and  air  cooled.  The  very  large  sorbitic  grains  should  be  noted  as  well  as  the 
very  small  proportion  of  free  ferrite. 

Oil  and  Water  Quenching  from  Annealing  Temperature.  —  As  already  explained 
only  steel  containing  very  little  carbon  may  be  quenched  in  oil  or  water  for  purposes 
of  annealing,  unless,  indeed,  the  double  treatment  soon  to  be  described  be  employed 
when  higher  carbon  steels  may  be  so  quenched.  The  structure  of  steel  containing 
O.K)  per  cent  carbon,  heated  to  !)50  deg.  and  quenched  in  water,  is  shown  in  Figure 
229  while  in  Figure  230  is  seen  the  structure  of  steel  containing  0.20  per  cent  carbon 


240 


CHAPTER  XV  — THE  ANNEALING  OF  STEEL 


quenched  in  oil  from  a  temperature  of  850  deg.  Rapid  cooling  through  the  range  did 
not  prevent  the  separation  of  the  bulk  of  the  large  amount  of  excess  ferrite  present 
in  these  steels,  hence  their  softness  and  ductility  even  after  quenching.  They  are, 
however,  somewhat  stronger  and  more  elastic  than  similar  steels  more  slowly  cooled, 
(1)  because  they  contain  a  somewhat  smaller  proportion  of  soft  free  ferrite,  (2)  be- 
cause the  free  ferrite  they  contain  has  crystallized  into  smaller  grains,  and  (3)  because 
their  carburized  constituent  is  sorbitic  or  even  nmrtensitic1  rather  than  pearlitic. 

Double  Annealing  Treatment.  —  It  has  been  stated  that  the  most  effective  way 
of  retaining  in  the  cold  the  very  fine  structure  acquired  by  steel  in  passing  through 
its  critical  range  consisted  in  cooling  it  very  rapidly  as  soon  as  it  emerged  from  that 
range,  as  for  instance  by  quenching  it  in  water.  This  treatment,  however,  unless  the 
metal  contains  very  little  carbon,  hardens  the  steel  and  deprives  it  of  ductility,  where- 


Fig.  229. —  Steel.  Carbon  0.10  per  cent.  Mag- 
nified 100  diameters.  Heated  to  950  dog.  C.  and 
quenched  in  water.  (Boylston.) 


Fig.  2:ilt.  —Steel.  Carbon  0.20  per  cent.  Mag- 
nified 100  diameters.  Heated  to  850  deg.  C. 
and  quenched  in  oil.  (Boylston.) 


as  annealed  steel  should  not  be  very  hard  and  should  possess  much  ductility.  If 
this  fine  grained  but  hard  steel,  however,  be  reheated  to  a  temperature  close  to  but 
below  its  critical  range,  say  to  from  500  to  650  deg.  C.,  it  loses  its  hardness  but  re- 
tains its  fine  structure  and  again  becomes  ductile  (IV,  Fig.  217). 

The  double  treatment  outlined  above  fulfils  admirably  the  aims  generally  sought 
in  annealing,  namely,  the  production  of  a  very  fine  structure  possessing  high  strength 
and  elastic  limit  with  fair  ductility,  in  other  words  toughness  and  high  resistance  to 
wear,  to  shock,  and  to  fatigue.  The  change  of  structure  taking  place  on  heating  hard- 
ened steel  close  to  the  lower  limit  of  its  critical  range  will  be  considered  at  some  length 
in  Chapter  XVII.  It  will  suffice  to  note  here  that  the  metal  passes  from  a  fine  mar- 
tensitic  or  troostitic  condition  (the  ordinary  condition  of  well-hardened  steel)  to  an 
equally  fine  sorbitic  condition,  possessing  in  a  high  degree  the  physical  properties 

1  Martensite  is  the  ordinary  constituent  of  steel  hardened  by  quenching.  It  is  hard  and  deprived 
of  ductility. 


CHAPTER   XV  — THE   ANNEALING   OF   STEEL  241 

desired.     The  first  heating  is  sometimes  called  "grain  refining"  treatment  and  the 
second  "toughening"  treatment. 

The  quenching  of  a  piece  of  steel  from  above  its  critical  range,  while  simple  enough 
in  the  case  of  very  mild  steel,  presents  increasing  difficulties  as  the  carbon  increases. 
It  should  be  conducted  with  care  and  intelligence  and  only  by  experts.  Steel  con- 
taining but  little  carbon,  say  not  over  0.30  per  cent,  may  be  quenched  in  water, 
others  should  be  quenched  in  oil.  The  ( 'ommittee  on  Heat  Treatment  of  the  Amer- 
ican Society  for  Testing  Materials  recommends,  in  order  to  lessen  the  danger  of 


Fig.  231.  —  Steel.  Carbon  0.60  per  cent.  Magnified  150  diameters. 
Heated  to  850  deg.  C.,  quenched  in  water,  reheated  to  000  deg., 
and  cooled  in  air. 


cracking,  that  the  object  be  removed  from  the  oil  or  water  bath  before  its  tempera- 
ture has  fallen  below  160  deg.  C.,  or  in  any  event  below  100  deg.,  and  that  the  second 
treatment  be  applied  within  a  few  hours  after  the  quenching,  preferably  without  ever 
allowing  the  piece  to  cool  below  100  deg.  and  certainly  not  below  20  degrees.  The 
final  properties  of  the  steel  will  depend  upon  the  temperature  of  the  second  heating; 
the  higher  that  temperature  the  softer  and  more  ductile  will  it  be,  but  also  the  less 
strong  and  elastic.  For  great  strength,  high  elastic  limit,  and  little  ductility  reheating 
to  400  or  500  deg.  should  be  applied,  while  for  great  ductility,  at  tjie  sacrifice  of  consid- 
erable strength,  the  reheating  should  be  carried  to  some  700  deg.  For  intermediate 
tensile  strength,  elastic  limit,  and  ductility  such  as  are  desired  in  the  majority  of 
cases,  the  temperature  of  the  second  treatment  should  be  between  550  and  650  deg.  C. 
While  from  purely  theoretical  considerations  it  might  be  argued  that  the  rate  of  cool- 


242 


CHAPTER   XV  — THE   ANNEALING   OF   STEEL 


ing  from  this  second  treatment  is  immaterial,  there  is  little  doubt  but  that  the  strength 
of  the  steel  increases  somewhat  and  its  ductility  decreases  with  the  rapidity  of  cool- 
ing. This  cooling  may  be  performed  in  the  furnace,  in  air,  in  oil,  or  in  water. 

The  double  annealing  treatment  described  in  the  foregoing  paragraphs  was  first 
suggested  by  Wallerant  of  the  Creusot  Steel  Works,  France.  It  was  also  described 
by  Andre1  Le  Chatelier  and  adopted  by  the  French  navy.  Its  use  is  now  general 
when  high  physical  requirements  are  to  be  met. 

In  Figure  231  is  shown  the  structure  of  steel,  containing  some  0.50  per  cent  carbon, 
after  double  annealing.  The  fineness  of  the  structure  should  be  noted  as  well  as  the 


Fig.  232.  —  Steel.  Eutectoid.  Magnified  412  diameters.  Heated  to  800  deg. 
C.  and  slowly  cooled  in  furnace.  (C.  C.  Buck,  Correspondence  Course 
student.) 


lack  of  laminations  and  the  absence  of  free  ferrite.  This  steel  is  composed  wholly  of 
finely  divided  sorbite. 

Annealing  Eutectoid  Steel.  —  While  the  mechanism  of  the  structural  changes 
taking  place  on  annealing  steel  has  been  made  clear  in  the  preceding  pages,  it  may 
not  be  without  interest  to  consider  further  and  in  succession  the  annealing  of  eutec- 
toid,  hypo-eutectoid,  and  hyper-eutectoid  steel,  as  these  three  types  of  steels  have 
different  structures  and  their  annealing  involves  different  structural  changes. 

Slowly  cooled  eutectoid  steel  is  composed  wholly  of  pearlite  which,  upon  being 
heated  through  the  single  critical  point  of  the  metal,  namely  Ac3.2.i,  is  converted  into 
a  solid  solution  (austenite).  The  grains  of  this  austenite  are  very  fine  as  the  steel 
emerges  from  its  range  and  they  are  kept  from  growing  by  preventing  the  steel  from 
reaching  a  higher  temperature.  On  cooling  through  Ars.2.i  the  metal  again  becomes 
pearlitic  if  it  be  given  time,  as  for  instance  in  cooling  in  the  furnace,  while  it  becomes 
sorbitic  if  cooled  more  quickly,  as  for  instance  in  air  in  the  case  of  small  objects. 


CHAPTER   XV  —  THE   ANNEALING   OF   STEEL  243 

Should  the  steel  be  quenched  in  water  or  oil  from  the  annealing  temperature  and  then 
reheated  near  but  below  the  point  Acs.2.i,  the  finely  martensito-troostitic  structure 
produced  by  quenching  from  above  the  range  is  converted  into  very  fine  sorbite. 

It  has  been  explained  in  Chapter  XIII  that,  for  like  treatments,  the  structure  of 
eutectoid  steel  is  finer  than  that  of  either  hypo-eutectoid  or  hyper-eutectoid  steel  and 
this  holds  true  in  the  case  of  annealed  samples,  although  the  difference  may  not  be 
noticeable  when  comparing  the  structure  of  eutectoid  steel  with  that  of  hyper-eutec- 
toid steel  containing  but  a  slight  excess  of  free  cementite. 

In  Figure  232  is  shown  the  structure  of  eutectoid  steel  heated~to~800  deg.  C.  and 
slowly  cooled  in  the  furnace.  It  is  made  up  of  well-developed  pearlite.  The  struc- 


Fig.  233.  —  Steel.  Eutectoid.  Magnified  720  diam- 
eters. Heated  to  825  deg.  C.,  quenched  in  oil, 
reheated  to  050  dog.,  and  cooled  in  air.  (Boylston.) 

ture  of  the  same  steel,  quenched  in  oil  at  825  deg.,  reheated  to  650  deg.,  and  cooled 
in  air,  is  exhibited  in  Figure  233.  The  metal  is  now  composed  of  fine  grained  sorbite. 
Annealing  Hypo-Eutectoid  Steel.  —  Slowly  cooled  hypo-eutectoid  steel  is  an  ag- 
gregate of  pearlite  and  free  ferrite.  On  being  heated  through  its  critical  range,  as 
soon  as  the  point  Aci  is  reached,  the  pearlite  is  bodily  converted  into  austenite,  while 
the  ferrite  still  remains  free.  On  further  heating,  however,  it  begins  to  be  absorbed 
by  austenite,  its  absorption  being  completed  as  the  metal  emerges  from  its  Ac3  point. 
Above  Acs  the  steel  is  composed  wholly  of  homogeneous  austenite.  On  cooling 
through  the  critical  range  unless,  indeed,  the  cooling  be  very  rapid  and  sufficient 
carbon  be  present,  ferrite  is  again  liberated  in  amount  proportional  to  the  slowness 
of  the  cooling  up  to  the  maximum  quantity  consistent  with  the  carbon  content  in 
the  steel.  If  the  cooling  be  very  slow  then,  for  instance  in  the  furnace,  the  totality 
of  the  excess  ferrite  will  be  rejected  and  the.  residual  austenite  converted  into  well- 
defined  pearlite  (Figs.  221  and  226),  while  if  the  cooling  be  more  rapid,  for  instance  in 
air  in  the  case  of  small  objects  or  in  oil  with  larger  ones,  a  portion  only  of  the  excess 


244 


CH.U'TKK   XV  — THK   ANNKAMNC,    OF   STKKL 


ferrite  is  liberated  while  the  residual  austenite  is  converted  into  sorbite  (Figs.  222 
and  227).'  The  liberation  of  ferrite  taking  plaee  during  the  slow  cooling  of  hypo- 
eutectoid  steel  coarsens  its  structure  and  is  the  chief  reason  why  annealed  hypo- 


^^1$=%,. 

.-.'  $feu»c?^^4&> 

^L-.  ,     --  frj      -^  >., 


Fig.  234.  —  Steel.  Hypo-eutectoid.  0.20  per 
cent  carbon.  Annealed.  Magnified  100  di- 
ameters. (W.  .1.  Burger,  Correspondence 
Course  student.) 


Kip.  235.  —  Steel.  Carbon  1.43  per  cent.  Magnified  ";00  diameters. 
Heated  above  critical  range  and  slowly  cooled  in  furnace.  (Boyn- 
ton  in  the  author's  laboratory.) 

eutectoid  steel  cannot  have  as  fine  a  structure  as  annealed  eutectoid  steel.  Howe 
further  contends  that  as  hypo-eutectoid  steel  is  heated  from  Act  to  Ac3  a  new  crystal- 
line growth  takes  place  which  is  the  coarser  the  greater  the  distance  between  A,  and 


CHAPTER   XV  — THE   AX.XEALIXC,    OF   STEEL 


245 


A:),  that  is  the  less  carbon  in  the  steel,  so  that  by  the  time  the  old  structure  has  been 
obliterated,  i.e.  at  Acs,  a  new  grain  has  formed  which  is  an  additional  reason  why  the 
structure  of  hypo-eutectoid  steel  cannot  be  refined  to  the  same  extent  as  that  of 
eutectoicl  steel. 

The  structure  of  hypo-eutectoid  steel  after  double  annealing  has  been  shown  in 
Figure  231.  The  rapid  cooling  through  the  range  prevented  the  liberation  of  ferrite, 
while  the  second  treatment  produced  sorbite,  but  this  sorbite  is  not  as  fine  grained 
as  that  produced  in  eutectoid  steel  by  similar  treatment. 

An  additional  illustration  of  the  fine  structure  that  can  be  imparted  to  hypo- 
eutectoid  steel  by  annealing  is  given  in  Figure  234  in  the  case  of  steel  containing 
0.20  per  cent  carbon. 


Fin- -:>'>.       Steel.    Carbon  1.25  per  cent.    Magnified  670  diameters.    Heated 

to  SOO  deg.  C.  and  quenched  in  oil,  reheated   to  (>()()  deg.  and  air  cooled. 
(C.  C.  Buck,  Correspondence;  Course  student.)     MO 


Annealing  Hyper-Eutectoid  Steel.  —  Slowly  cooled  hyper-eutectoid  steel  is  an 
aggregate;  of  pearlite  and  free  cementite.  On  heating  it  through  its  critical  range,  i.e. 
through  its  Ac;).->.i.  and  Ac,.,n  points,  pearlite  is  converted  into  austenite  at  the  lower 
point  and  this  austenite  absorbs  the  free  cementite  as  the  metal  is  further  heated 
from  A(VJ.I  to  Ac,.,,,.  At  Ac,.,,,  the  absorption  is  complete  and  the  metal  composed 
entirely  of  austenite.  On  cooling  through  the  range,  if  time  be  given,  as  for  instance 
in  cooling  in  the  furnace,  the  full  proportion  of  free  cementite  is  again  liberated  and 
the  residual  austenite  converted  at  Ar:).2.i  into  clearly  laminated  pearlite  (Fig.  235). 
If  the  cooling  be  more  rapid,  as  for  instance  in  cooling  small  pieces  in  air,  a  portion 
only  of  the  free  cementite  is  set  free,  while  the  residual  austenite  is  converted  into 
sorbite.  This  setting  free  of  cementite,  like  the  liberation  of  ferrite  in  hypo-cirtec- 
toid  steel,  coarsens  the  structure.  The  coarsening  influence  of  free  cementite,  how- 


246  CHAPTER   XV— THE   ANNEALING    OF   STEEL 

ever,  is  far  from  being  as  marked  as  that  of  free  ferrite,  chiefly  because  free  cementite 
is  generally  present  in  much  smaller  proportions.  Steel  containing  as  much  as  1.50 
per  cent  carbon,  for  instance,  contains  but  11.50  per  cent  of  coarsening  cementite, 
while  steel  with  0.40  carbon  contains  52  per  cent  of  coarsening  ferrite. 

In  Figure  236  is  shown  the  structure  of  hyper-eutectoid  steel  subjected  to  the 
double  annealing  treatment,  which  resulted  in  the  production  of  fine  grained  sorbito. 

For  the  purpose  of  annealing  hyper-eutectoid  steel,  it  is  seldom  advisable  to  heat 
it  much  above  its  lower  critical  point  Acs.j.i,  because  while  at  Acm  the  free  cementite 
would  be  completely  reabsorbed  and,  through  relatively  quick  cooling,  could  be 
prevented,  to  a  certain  extent  at  least,  from  again  separating,  the  coarseness  of 
structure  resulting  from  so  high  a  temperature  would  generally  more  than  offset  the 
gain  resulting  from  the  presence  of  a  smaller  proportion  of  free  cementite. 


Fig.  237.  —  Steel  wire.     0.08  per  cent  car-  Fig.  238.  —  Steel  wire.     0.08  per  cent   car- 

bon.    Cold  drawn.     Magnified  100  diam-  bon.     Cold  drawn  and  reheated  below  its 

eters.     (E.  H.  Peirce.)  critical  range.     Magnified   100  diameters. 

(E.  H.  Peirce.) 

Annealing  of  Cold  Worked  Steel.  —  It  has  already  been  mentioned  that  the  cold 
working  of  steel,  that  is,  the  application  of  severe  compression  or  tension  when  its 
temperature  is  below  its  critical  range,  and  more  specifically  at  atmospheric  tem- 
perature, results  in  a  marked  distortion  of  its  structural  elements,  a  decided  elonga- 
tion for  instance  if  the  steel  is  subjected  to  tension  as  in  wire  drawing  (Fig.  237,  also 
Figs.  215  and  216,  Chapter  XIV),  and  that  such  distortion  causes  a  great  increase  of 
tenacity  and  hardness  but  that  it  lowers  the  ductility  decidedly,  eventually  produc- 
ing brittleness.  It  has  also  been  stated  that  many  other  properties  of  steel  are  deeply 
affected  and  that  this  action  of  cold  working  is  ascribed  by  some  to  the  formation  of 
a  certain  amount  of  amorphous  iron  (Chapter  XIV). 

When  it  is  desired  to  increase  the  ductility  of  cold  worked  steel  in  order  that  it 
may  be  subjected  to  additional  drafting  or  because  the  uses  to  which  it  is  to  be  put 
call  for  greater  ductility  and  softness,  the  metal  should  be  subjected  to  suitable  heat 
treatment.  In  the  case  of  cold  worked  hypo-eutectoid  steel  which  is  composed  of 
elongated  particles  of  ferrite  and  pearlite  it  is  sufficient  in  order  to  restore  its  due- 


CHAPTER    XV       TIIK   ANNEALING   OF   STEEL  247 

tility  to  heat  it  to  a  temperature  varying  between  550  and  600  cleg.  C.  and,  therefore, 
considerably  below  its  critical  range  (see  Fig.  238).  This  is  especially  so  when  the 
carbon  content  does  not  exceed  0.40  per  cent  and  when  therefore  the  metal  contains 
a  large  amount  of  free  ferrite.  It  has  been  stated  by  some  that  520  deg.  C.  marks 
the  temperature  which  should  be  reached  in  order  to  produce  the  desired  results  in 
very  low  carbon  steel.  This  heating  of  cold  worked  steel  below  the  critical  range  to 
increase  its  ductility  is  sometimes  called  at  the  works  "process"  or  "works"  annealing 
(J.  T.  Tinsley) .  The  treatment  causes  the  free  ferrite  to  recrystallize  in  its  normal 
polyhedral  pattern  (Fig.  238)  or,  if  we  believe  in  the  existence  of  amorphous  iron  in 
cold  worked  steel,  the  return  of  that  amorphous  iron  to  the  crystalline  condition  and 
hence  decreased  hardness  and  strength  and  increased  ductility.  It  should  be  noted, 
however,  that  the  elongated  particles  of  pearlite  remain  elongated  (Fig.  238),  the 


Fig,  239.  —  Steel  wire.     Cold  drawn  and  re-  Fig.   240.  —  Patented  steel  wire.     0.85   per 

heated  above  its  critical  range.     Magnified  cent  carbon.     Magnified  1000  diameters. 

100  diameters.     (Boylston.)  (E.  H.  Peirce.) 

ferrite  particles  alone  being  affected  by  the  treatment.  When  maximum  softness  is 
required  the  cold  worked  steel  must  be  annealed  by  heating  it  above  its  critical  range 
followed  by  slow  cooling.  This  treatment,  of  course,  removes  the  distortion  of  the 
pearlite  as  well  as  of  the  ferrite  (Fig.  239)  since  both  constituents  above  the  critical 
range  form  a  homogeneous  solid  solution.  In  wire  mills  this  is  known  as  "dead  soft" 
annealing  (Tinsley) . 

When  it  is  desired  to  produce  wires  having  a  very  high  yield  point  and  very  great 
tenacity  (300,000  to  400,000  Ibs.  per  sq.  in.)  while  retaining  considerable  toughness, 
the  heat  treatment  is  so  conducted  as  to  produce  sorbite  to  the  practical  exclusion  of 
pearlite,  free  ferrite,  or  free  cementite,  and  is  generally  applied  to  steel  wires  contain- 
ing some  0.35  to  0.85  per  cent  carbon  (Tinsley).  The  process  is  known  as  "patent- 
ing" and  the  product  as  "patented"  wire.  To  produce  sorbite  the  cooling  through 
the  critical  range  should  be  relatively  rapid  but  not  so  rapid  as  to  cause  the  retention 
of  martensite  or  even  troostite  lest  the  steel  be  brittle,  nor  should  it  be  slow  enough 
to  permit  the  formation  of  pearlite  since  this  would  imply  decreased  tenacity.  Ac- 
cording to  Tinsley,  patenting  in  practice  is  usually  conducted  as  a  continuous 


248  CHAPTER    XV  — THE   ANNEALING   OF   STEEL 

operation,  the  wire  being  led  through  the  heated  tubes  of  a  furnace  and  cooled  by 
being  brought  into  the  air  or  into  a  bath  of  molten  lead  comparatively  cooled  but 
seldom  below  700  deg.  F.  (367  deg.  C.).  The  structure  of  patented  wire  is  shown  in 
Figure  240. 

Tinsley  writes  that  by  a  proper  combination  of  drafting  and  patenting  it  is  possi- 
ble to  obtain  music  wire  from  a  0.70  per  cent  carbon  steel  which  will  have  a  tensile 
strength  of  400,000  Ibs.  per  sq.  in.,  and  be  sufficiently  tough  to  be  wrapped  about  itself 
without  breaking  and  be  swaged  flat  to  one  half  its  original  thickness  without  split- 
ting. The  same  author  also  states  that  although  it  might  at  first  be  supposed  that 
wire  annealed  to  a  pearlitic  condition  because  of  its  greater  softness  and  ductility 
should  withstand  drafting  to  a  greater  extent  than  patented  (sorbitic)  wire  the  facts 


Fig.  241.  —  Steel.  Cast.  Carbon  0.30  per  cent. 
Magnified  100  diameters.  Annealed.  (W.  .1. 
Burger,  Correspondence  Course  student.) 


are  that  owing  to  the  rapid  loss  of  ductility  characteristic  of  a  pearlitic  structure  it 
will  not  withstand  drawing  to  anywhere  near  the  same  degree  as  will  the  patented 
structure. 

Annealing  Steel  Castings.-  It  has  been  mentioned  that  the  very  coarse  struc- 
ture of  steel  castings,  called  "ingotism"  by  Howe,  was  not  as  readily  refined  as  the 
structure  of  steel  forgings,  its  satisfactory  annealing  often  necessitating  prolonged 
heating  slightly  above  the  range  or  short  heating  to  temperatures  considerably  higher 
than  the  critical  range.  Notwithstanding  their  greater  resistance  to  the  annealing 
treatment,  successfully  annealed  castings  may  possess  physical  properties  fairly  equal 
to  those  of  forgings.  In  Figure  241  is  shown  the  structure  of  cast  steel  containing 
some  0.30  per  cent  of  carbon  and  properly  annealed.  The  presence  of  a  relatively 
small  amount  of  free  ferrite  will  be  noted.  When  highly  magnified  the  carbon-hold- 
ing constituent  should  have  a  sorbito-pearlitic  appearance. 

The  Committee  on  Heat  Treatment  of  the  American  Society  for  the  Testing  of 
Materials  recommends,  for  the  purpose  of  annealing,  heating  carbon-steel  castings 


l''ig.  244.  —  Heated  tn  S75  de-;.  ( '.  fur  five  hours 
and  ail'  i-onlnl. 


Fig.  243. —  Heated  to  S75  licit.  C.  and  furnace  cooled. 


Fig.  245.  —  Heatod  to  875  dcjj.  C.  and  air  cooled. 


l-'is;.  L'lti.  -     llcati'il  in  11(1(1  ili'K.  C.  and  air  muled,  rt-  Fig.  247.  —  Heated  to  875  deg.  C.  and  quenched  in 

heated  to  875  ilc;;.  ( '.  and  air  rooli'd.  water,  reheated  to  660  deg.  C.  and  quenched  in  oil. 


Aimpaling  of   cast   sled   rout  Mining  0.30  ])<T    cent  carbon.     All  photomicrographs  are   magnified 

100  diainr'frrs.     (K.  S.  Simmons  in  the  author's  laboratory.) 

249 


250  CHAPTER   XV  —  THE   ANNEALING   OF   STEEL 

» 

slowly  and  uniformly  to  temperatures  varying-  with  the  carbon  content  of  the  steel 
approximately  as  follows: 

CARBON  PER  CENT  TEMPERATURES  DEG.  C. 
Up  to  0.16  925 

0.16  to  0.34  875 

0.35  to  0.54  850 

0.55  to  0.79  830 

The  castings  should  be  kept  at  the  maximum  temperature  a  sufficient  length  of 
time  to  ensure  the  refining  of  the  grain.  The  castings  should  be  cooled  (1)  slowly 
and  uniformly  in  the  furnace  when  it  is  desired  that  the  steel  shall  possess  the  maxi- 
mum softness  and  (2)  at  an  accelerated  rate,  when  it  is  desired  that  the  steel  possess 
rather  higher  tensile  strength  and  elastic  limit. 

The  rather  long  annealing  indicated  above  may  be  replaced  by  heating  the  cast- 
ings to  some  1200  deg.  C.  for  a  few  minutes,  cooling  in  the  air,  and  reheating  to  a 
temperature  slightly  above  Ac3.2.i,  Ac3.2,  or  Aci  (J.  H.  Hall). 

The  double'  treatment  used  with  such  satisfactory  results  in  annealing  steel  forg- 
ings  may  also  be  applied  to  castings  and  is  strongly  recommended  by  Hall,  especially 
when  it  is  desired  to  develop  high  resistance  to  shock,  maximum  strength,  and  elastic 
limit.  From  the  first  heating  which  should  be  but  slightly  above  Ac3.2.i,  Acs.2  or  Aci 
the  castings,  if  their  carbon  content  and  shape  permit  it,  should  be  quenched  in 
water,  or  if  too  high  in  carbon  or  of  such  shape  as  to  lead  to  cracking,  in  oil.  They 
should  then  be  reheated  to  from  640  to  680  deg.  C.  for  2  to  8  hours,  in  order  to  con- 
vert any  martensite  or  troostite  into  sorbite  and  to  relieve  cooling  stresses.  If  the 
carbon  content,  size,  and  shape  of  the  castings  are  such  that  there  is  danger  of  crack- 
ing when  quenched  in  water  or  even  in  oil,  they  should  be  cooled  in  air  or  in  an  air 
blast  and  reheated  to  from  660  to  720  deg.  C.  to  relieve  cooling  stresses.  If  the  cast- 
ings are  cooled  in  oil  or  in  water  it  is  best  to  remove  them  from  the  quenching  baths 
before  they  have  become  cold  and  they  should  not  be  placed  in  a  hot  furnace  or  re- 
heated rapidly  when  put  in  a  cold  furnace. 

The  structures  imparted  to  cast  steel  containing  0.30  per  cent  carbon  (Fig.  242) 
by  various  annealing  treatments  are  illustrated  in  Figures  243  to  247. 

Rate  of  Cooling  vs.  the  Structure  of  Steel.  —  An  attempt  has  been  made  in  Figures 
248  and  249  to  illustrate  graphically  the  mechanism  of  the  very  great  influence  ex- 
erted upon  its  structure  and  physical  properties  by  the  speed  with  which  steel  cools 
through  its  thermal  critical  range.  Steel  containing  some  0.30  per  cent  carbon  has 
been  selected  but  the  same  reasoning  would  apply  to  any  other  hypo-eutectoid  steel 
as  well  as  to  hyper-eutectoid  steel.  In  the  latter  case,  however,  free  cementite  in- 
stead of  ferrite  would  form,  if  time  be  given,  in  cooling  through  the  range. 

It  is  shown  in  Figure  248  that  while  the  steel  above  its  critical  range  is  composed 
entirely  of  austenite  and  while,  after  very  slow  cooling,  it  contains  nothing  but  fer- 
rite and  pearlite,  in  passing  through  the  range  the  transformation  of  the  solid  solu- 
tion austenite  into  the  ferrite-pearlite  aggregate  is  not  sudden  but  gradual,  several 
transition  constituents  being  formed,  namely  martensite,  troostite,  and  sorbite.  Sor- 
bite has  already  been  described  while  the  nature  of  martensite  and  troostite  will  be 
discussed  in  the  next  chapter.  It  follows  as  indicated  in  Figure  248  that  various 
types  of  structures  are  produced  theoretically  at  least  while  steel  cools  through  its 


CHAPTER   XV  — THE   ANNEALING   OF   STKKL 


251 


Cfittcaj 


700' 


6OO 


r        P 


Ferrito-Austenite-MiLCtensitic 

Ferrito-Martensitic 

Ferri  to-Troostito-Martensi  tic 

Ferri  to-Troosti  ti  c 

Ferri  to-Troosti to-Sorbi tic 

Ferrito-Sorbitic 

Ferri  to-  Pearlito-S<  trbitie 


—     Ferrito-Pearlitic 


Fig.  248.  —  Diagram  depicting  the  mechanism  of  the  structural 
transformation  in  steel  containing  0,30  per  cent  carbon  on  slow  cooi- 
ing  through  the  critical  range. 


Very  Slmv 
Cooling 


II 

Slow 
Cooling 


III 

Quick 
Cooling 


V 

Very  Quick 
Cooling 


Fig.  240.  —  Diagram  depicting  the  influence  of  the  rate  of  cooling  on  the  mechanism  of 
the  structural  transformation  taking  place  in  the  critical  range  in  steel  containing  0.20 
per  cent  carbon. 


252  CHAPTER   XV  — THE    AXXKALIXC1    OF   STEEL 

critical  range,  containing  besides  free  ferrite  either  (1)  austenite  and  martcnsiio, 
(2)  martensite  alone,  (3)  martensite  and  troostite,  (4)  troostite  alone,  (5)  troostite 
and  sorbite,  (6)  sorbite  alone,  or  (7)  sorbite  and  pearlite. 

In  Figure  249  it  is  intended  to  depict  the  possibility  of  retaining-  in  the  cold, 
through  suitable  regulation  of  the  cooling,  these  various  structures  and,  therefore, 
the  possibility  by  this  means  of  imparting  to  steel  many  different  physical  properties. 
As  cooling  is  accelerated,  first  some  sorbite  is  retained  together  with  pearlite,  then  in 
succession,  sorbite  alone,  sorbite  and  troostite,  etc.,  while  the  amount  of  free  ferrite 
separating  decreases.  The  metal,  therefore,  becomes  stronger,  harder,  and  less  duc- 
tile (1)  because  of  a  diminishing  proportion  of  soft,  ductile  ferrite  and  (2)  because  of 
tenacious  but  moderately  ductile  sorbite  replacing  weaker  but  more  ductile  pearlite. 
The  presence  of  troostite  and  martensite  is  an  indication  that  the  metal  has  cooled 
so  quickly  through  its  critical  range  that  it  is  hardened  or  at  least  partially  so  as 
explained  in  Chapter  XVI.  If  the  metal  is  needed  in  an  annealed  condition  it  should 
then  be  reheated  as  explained  in  the  foregoing  pages  to  slightly  below  its  critical 
range  in  order  that  the  troostite  and  martensite  may  be  converted  into  sorbite  (a 
tempering  process  as  explained  in  Chapter  XVII). 

Structure  vs.  Heat  Treatment.  —  Some  of  the  many  different  structures  pro- 
duced by  various  treatments  are  shown  in  a  convenient  form  in  Figures  250  and  2.~>1 
in  the  case  of  steels  containing  respectively  0.30  and  0.50  per  cent  carbon  and 
otherwise  of  good  commercial  quality.  The  treatments  corresponding  to  each  struc- 
ture are  indicated  in  the  accompanying  legenda. 

Spheroidizing  of  Cementite.  —  It  is  now  well  understood  that  on  slow  cooling 
through  the  critical  range  austenite  of  eutectoid  composition  is  converted  into  well- 
defined  pearlite  made  up  of  distinct  parallel  plates  or  lamella;  alternately  of  ferrite 
and  cementite,  while  in  hyper-eutectoid  steel  the  excess  cementite  tends  to  form  at 
the  boundaries  of  the  pearlite  grains  and  between  the  cleavage  planes  of  the  austen- 
ite. This  condition  of  the  cementite  of  pearlite  as  well  as  of  the  free  cementite  is 
not,  however,  final,  not  being  structurally  stable,  for  if  the  steel  be  kept  for  a  suffi- 
ciently long  time  at  a  temperature  but  slightly  below  its  critical  range,  preferably 
between  600  and  700  deg.  C.  the  cementite  shows  a  marked  tendency  to  collect  in 
the  form  of  rounded  particles  embedded  in  a  matrix  of  ferrite.  The  phenomenon 
has  been  called  "spheroidizing"  by  Howe,  or  when  restricted  to  the  pearlite-cemen- 
tite,  "divorcing,"  on  the  ground  that  it  constitutes  a  true  divorce  between  the  con- 
stituents of  pearlite,  namely  ferrite  and  cementite.  Pearlite  that  has  undergone 
spheroidizing  is  sometimes  called  "granular"  or  "globular"  pearlite. 

Both  hypo-  and  hyper-eutectoid  steels  are  reported  to  spheroidize  more  readily 
than  eutectoid  steel,  the  free  ferrite  or  cementite  probably  acting  as  nuclei.  Even 
eutectoid  steel,  hcjwever,  can  be  completely  spheroidized  if  the  heating  be  continued 
long  enough.  Howe  states  that  the  transformation  is  more  rapid  in  a  0.21  per  cent 
carbon  than  in  a  0.59  per  cent  carbon  steel.  It  seems  probable  that  sorbite  yields  to 
the  spheroidizing  treatment  more  readily  than  true  pearlite  from  which  it  follows  that 
for  the  purpose  of  spheroidizing  the  steel  should  first  be  made  sorbitic.  This  can  be 
done  by  relatively  quick  cooling  through  the  critical  range  or  as  later  explained 
(Chapter  XVI)  by  very  quick  cooling  through  the  range,  thereby  producing  marten- 
site  or  troostite,  followed  by  reheating  slightly  below  the  critical  range  which  is  also 
the  temperature  at  which  the  spheroidizing  operation  should  be  conducted. 

Howe  and  Levy  state  that  both  high  heating  and  long  heating  above  the  range 


CHAPTER   XV  — THK   AXXEALIXG    OF   STEEL 


253 


Fig.  250.  —  Various   st  ructures  of  steel   containing  0.30  per 
cent  carbon.     Magnified  about  30  diameters. 


8 
9 

10. 
11 
12. 


14. 
1.5. 
10. 


LKCKNUA   FOR   FHJUKE   250 

Cast  condition. 

Cast  and  improperly  annealed. 

Cast  and  properly  annealed. 

li^-in.  rolled  bar  heated  for  2  hours  at  1000  (leg.  C.  and  cooled  in  furnace. 

J^-in.  rolled  bar  heated  to  1000  deg.  C.,  cooled  in  furnace. 

IJ^-in.  round  rolled  bar  heated  to  1000  deg.  C.  and  cooled  in  furnace. 

K-in.  round  rolled  bar  heated  to  1000  deg.  C.  and  cooled  in  furnace. 

Yz-m.  round  rolled  bar  heated  to  900  deg.  C.  and  cooled  in  furnace. 

3/2-in.  round  rolled  bar  heated  to  900  deg.  C.  and  cooled  in  air. 

1  2-in.  round  rolled  bar  heated  to  900  deg.  C.  and  cooled  in  oil. 

'2-111.  round  rolled  bar  heated  to  !)()()  deg.  C.  and  cooled  in  water. 

Yy\\\.  round  rolled  bar  heated  to  900  deg.  C.  and  cooled  in  oil;  reheated  to  050  deg.  C.  and 

cooled  in  oil. 
\Vf\\\.  round  rolled  bar  heated  to  900  deg.  C.  and  cooled  in  oil;  reheated  to  650  deg.  C. 

and  cooled  in  oil. 

J^-in.  round  rolled  bar  finished  at  proper  temperature. 
114-in.  round  rolled  bar  finished  at  high  temperature. 
Cold  worked. 


254 


CHAI'TKH    XV  — THE   ANNEALING    OF   STEEL 


9 


Fig.  251.  —  Various    structures    of    steel    containing  0.50  per 
cent  carbon.     Magnified  about  30  dmi 


LKGENDA  FOR  FIGURE  251 


1.  Cast  steel. 

2.  Cast  steel  improperly  annealed. 

3.  Cast  steel  properly  annealed. 

4.  >2-in.  round  rolled  steel  cooled  from  1100  deg.  C.  in  the  furnace. 

5.  Yz-va.  round  rolled  steel  cooled  from  1000  deg.  C.  in  the  furnace. 

6.  %-in.  round  rolled  steel  cooled  from  900  deg.  C.  in  the  furnace. 

7.  J^-in.  round  rolled  steel  soaked  2  hours  at  900  deg.  C.  and  cooled  in  the  furnace. 

8.  y-f-m..  round  rolled  steel  cooled  from  900  deg.  C.  in  the  air. 

9.  J^-in.  round  rolled  steel  cooled  from  1000  deg.  C.  in  water. 

10.  K-in.  round  rolled  steel  cooled  from  900  deg.  C.  in  water. 

11.  J^-in.  round  rolled  steel  cooled  from  900  deg.  C.  in  oil. 

12.  J^-in.  round  rolled  steel  quenched  in  water  while  passing  through  critical  range. 

13.  Ji-in.  round  rolled  steel  quenched  in  oil  from  850  deg.  C.;  reheated  to  600  deg.  C.  and 

quenched  in  oil. 

14.  }^-in.  round  steel  finished  at  proper  temperature. 

15.  Large  forging  showing  finishing  at  too  high  a  temperature. 

16.  Cold  worked. 


CHAPTER   XV  — THE   ANNEALING   OF   STEEL 


255 


retard  the  divorcing  of  pearlite.  According  to  Belaiew  the  patterns  of  Damascus  steel 
and  its  remarkable  physical  properties  are  probably  caused  by  a  divorcing  of  ferrite 
and  comentite  in  macroscopically  visible,  masses  produced  by  prolonged  exposure  to 


Fig.  252.  —  Steel.  Carbon  0.85  per  cent. 
Magnified  1000  diameters.  Spheroidized 
eemontite.  (E.  H.  Peiroe.) 


Fig.  253.  —  Steel.  Carbon  1.10  per  cent.  Mag- 
nified 1000  diameters.  Spheroidized  cemen- 
tite.  (W.  J.  Burger,  Correspondence  student.) 


Fig.  254.  —  Steel.  Carbon  1.24  per  cent. 
Magnified  1000  diameters.  Spheroidized 
cementite.  (Osmond.) 


a  temperature  not  exceeding  redness.  The  spheroidizing  treatment  greatly  decreases 
the  strength  and  elastic  limit,  while  increasing  the  ductility  and  softness  and  also 
the  resistance  to  wear  (Abbott),  these  effects  being  more  marked  in  high  than  in 
low  carbon  steels. 

Steel  in  the  spheroidized  condition  is  shown  in  Figures  252  to  254. 


256 


CHAPTER   XV  —  THE   ANNEALING   OF   STEEL 


Skoicin;/  tlm  Properties  of  Pearlite  and  its  Decomposition  Product. 
Fe.jC  repr&ieittetl  Black. 


Mechanical 1  Properties  of  M.crostructure. 

Mass. 


Segregation  Stages. 


Maximum  tensile  stress 
about  70  tons  per  square 
inch.  Elongation  on  2 
inches =about  10  per  cent. 


IST  PHASE. 
' '  Sorbitic ' '    !  pearlite    with 


emulsified     Fe3C. 
(lark  on  etching. 


Verv 


Maximum  tensile  stress 
about  55  tons  per  square 
inch..  Elongation  on  2 
inches=aboutl5percem. 


2xi)  PHASE. 

Normal  pearlite  with  semi- 
segregated  FejC.  Dark 
on  etching. 


Maximum  tensile  stress 
about  35  tons  per  square 
inch.  Elongation  on  2 
inches=about  5  per  cent 


SRD  PHASE. 

Laminated  pearlite  with 
completely  segregated 
Fe;)C.  Exhibiting  a  play 
of  gorgeous  colours  when 
lightly  etched. 


Maximum  tensile  stress 
about  30  tons  per  square 
inch. 


4TH  PHASE. 

Laminated  pearlite  passing 
into  massive  Fe3C  and 
ferrite. 


NOTE. — It  is  important  to  remember  tliat  in  a  single  section  of  sleel  two  or  even  all 
three  phases  of  pearlite  may  be  observed  in  juxtaposition  gradually  merging  into  each 
other. 


Fig.  255. —  (Arnold). 


CHAPTER   XV  —  THE   ANNEALING   OF   STEEL  257 

Varieties  of  Pearlite.  —  From  the  foregoing  it  will  be  evident  that  several  varie- 
ties of  pearlite  are  to  be  considered  and  that  the  physical  properties  of  steel  will  de- 
pend greatly  upon  the  character  of  the  pearlite  it  contains.  Arnold  considers  four 
varieties  of  pearlite  which  are  well  illustrated  in  Figure  255.  His  first  phase,  which 
he  calls  "sorbitic"  pearlite,  is  generally  called  sorbite  by  other  writers.  The  char- 
acter and  physical  properties  of  sorbite  have  been  described,  as  well  as  some  of  the 
conditions  necessary  to  its  formation.  His  second  and  third  phases,  to  which  he  gives 
the  names  respectively  of  "normal"  and  "laminated"  pearlite,  are  both  true  pearlite, 
the  thicker  lamellae  of  the  latter  being  due  to  a  slower  cooling  through  the  critical 
range.  His  fourth  phase  is  pearlite  in  the  process  of  spheroidizing. 

It  is  now  understood  that  true,  i.e.  clearly  laminated,  pearlite  only  has  a  con- 
stant composition,  being  in  this  respect  similar  to  all  eutectic  and  eutectoid  alloys 
as  explained  in  Chapter  XXV.  In  hypo-eutectoid  steel  sorbite  generally  contains 
more  ferrite  and,  therefore,  a  smaller  percentage  of  carbon  than  pearlite,  while  in 
hyper-eutectoid  steel  it  generally  contains  more  cementite,  hence  a  larger  per- 
tentage  of  carbon.  Moreover,  while  in  true  pearlite  the  carbon  is  probably  wholly 
present  as  crystallized  Fe3C  (cement  carbon),  in  sorbite  a  part  of  it  remains  dis- 
solved (hardening  carbon)  and  it  is  probably  to  the  presence  of  carbon  in  solution 
that  sorbite  owes  its  greater  strength  and  its  decreased  ductility. 

Graphitizing  of  Cementite.  —  It  will  be  explained  in  another  chapter  that  the 
carbide  Fe3C  (cementite)  is  not  the  most  stable  form  that  can  be  assumed  by  carbon 
when  alloyed  with  iron.  It  will  be  shown  that  cementite  tends  to  break  up  into  iron 
and  graphite  according  to  the  reaction 

Fe3C     =     3Fe     +     C 

i  i  I 

cementite     ferrite     graphite 

and  that  the  graphite  form  is  the  final  stable  condition  of  carbon.  This  graphitizing 
tendency  of  cementite  remains  latent  unless  the  conditions  be  favorable  to  its  activity. 
These  conditions  are  (1)  long  exposure  to  a  temperature  exceeding  the  critical  range 
and  slow  cooling,  (2)  the  presence  of  much  carbon,  and  (3)  the  presence  of  silicon  or 
of  some  other  elements  exerting  a  similar  influence.  It  will  be  shown  that  this  ten- 
dency of  cementite  to  be  converted  into  graphite  and  iron  is  responsible  for  the  pro- 
duction of  so-called  malleable  castings  and,  probably,  also  for  the  production  of  gray 
cast  iron.  In  the  case  of  steel,  because  of  the  relatively  small  amount  of  carbon 
present  (not  exceeding  1.75  or  at  the  most  2  per  cent),  the  graphitizing  tendency  is 
slight.  Long  exposure  of  hyper-eutectoid  steel,  especially  if  it  contains  more  than 
one  per  cent  carbon,  however,  to  a  temperature  exceeding  its  critical  range,  is  always 
likely  to  produce  a  small  amount  at  least  of  graphitic  carbon  greatly  impairing 
thereby,  if  not  ruining,  the  metal.  An  instance  of  graphite  formation  in  high  carbon 
steel  is  shown  in  Figures  256  and  257.  There  is  little  doubt  but  the  free  cementite 
present  in  hyper-eutectoid  steel,  and  formed  as  the  steel  cools  from  its  Arcm  to  its 
Ar3.2.i  point,  is  more  readily  converted  into  graphite  than  the  cementite  included  in 
the  pearlite.  Once  the  graphitizing  is  started,  however,  it  may  be  carried  to  comple- 
tion and  include  the  whole  of  the  cementite  present.  This  indeed  is  what  happens  in 
certain  grades  of  malleable  cast  iron  and  of  gray  cast  iron  which  contain  practically 
the  totality  of  their  carbon  in  the  form  of  graphite.  The  presence  of  some  free  cemen- 


258 


CHAPTER  XV  — THE  ANNEALING  OF  STEEL 


:••;, 

-  l'i^ftt    ''i\SS'->^WMX^^|-^;:j:  S|S 

" 


Fig.  256.  —  Steel.  Carbon  1.25  per  cent.  Magnified  150  diameters.  An- 
nealed five  hours  at  830  deg.  C.  (C.  C.  Buck.  Correspondence  Course 
student.) 


Fig.  257.  —  Steel.    Carbon  1.25  per  cent.     Magnified  670  diameters.    An- 
nealed five  hours  at  830  deg.  C.     (C.  C.  Buck,  Correspondence  Course 

student.) 


CHAPTER  XV  — THE   ANNEALING   OF   STEEL 


259 


tite  appears  to  be  necessary  to  start  the  graphitizing,  which  would  explain  why  it 
does  not  take  place  in  hypo-eutectoid  steel.  The  author  at  least  never  had  a  case 
of  graphite  formation  in  such  steels  brought  to  his  attention  nor  was  he  ever  able  to 
produce  graphite  in  hypo-eutectoid  steel. 

Over  Heating.  —  Long  exposure  of  steel  to  a  high  temperature  necessarily  de- 
velops a  very  coarse  structure  and  the  metal  is  then  said  to  have  been  overheated. 
Stead  describes  the  coarse  crystalline  structure  of  overheated  steel  containing  from 
0.20  to  0.50  per  cent  carbon  as  triangular  arrangements  of  ferrite  and  pearlite,  and 
that  of  steel  containing  from  0.50  to  0.70  per  cent  carbon  as  large  ferrite  cell  walls 
and  offshoots  of  ferrite  penetrating  the  pearlite  (Fig.  258).  Overheating  should  not 
be  confounded  with  burning:  overheated  steel  can  be  readily  restored  to  a  normal 


Fig.  258.  —  Steel.    Carbon  about  0.50  per  cent.     Heated  to  about 
1100  deg.  C.     (P.  J.  Neely,  Correspondence  Course  student.) 


condition  by  merely  heating  above  the  critical  range,  and  is  neither  red  short  nor  cold 
short,  while  burnt  steel  is  extremely  brittle. 

Burnt  Steel.  —  When  high  carbon  steel  is  heated  to  a  temperature  approaching 
its  melting-point,  it  becomes  extremely  red  short  as  well  as  cold  short  while  its  frac- 
ture becomes  very  coarse  and  shiny.  The  steel  in  such  condition  is  said  to  be  burnt. 
It  was  generally  believed  that  the  red  shortness  of  burnt  steel  could  not  be  cured 
short  of  remelting,  but  Stead  now  insists  that  if  burnt  steel  be  allowed  to  cool  com- 
pletely and  is  then  reheated  to  a  proper  temperature  it  can  be  rolled  and  forged  as 
well  as  if  it  had  not  been  burnt.  It  is  held  by  some  that  the  burning  of  steel  is  due  to 
the  evolution  of  gases  under  the  influence  of  a  high  temperature,  chiefly  carbon 
monoxide,  resulting  from  atmospheric  oxygen  finding  its  way  through  the  pores  of 
the  metal  and  combining  with  some  of  the  carbon,  although  according  to  Howe  other 
occluded  gases,  such  as  hydrogen  and  nitrogen,  may  also  contribute.  These  gases 
force  the  crystalline  grains  apart,  destroying  their  cohesion,  hence  the  brittleness  of 
the  metal.  Oxidized  membranes  are  also  frequently  found  surrounding  some  of  the 


260  CHAPTER   XV  — THE   ANNEALING   OF   STEEL 

grains,  their  presence  readily  explaining  the  impossibility  of  restoring  burnt  steel  by 
forging  since  they  would  prevent  the  welding  of  adjacent  grains.  Instances  of  the 
structure  of  burnt  steel  are  shown  in  Figures  259  and  260.  Howe  defines  burning  as 
being  "a  mechanical  separation  of  the  grains  on  extreme  overheating."  Some  writers 
have  argued,  apparently  on  good  ground,  that  burning  will  not  take  place  unless  the 
steel  has  been  heated  to  so  high  a  temperature  that  it  has  actually  begun  to  melt,  the 
explanation  being  perfectly  consistent  with  the  well-known  fact  that  high  carbon 
steel  burns  much  more  readily  than  low  carbon  steel.  To  make  the  matter  clear  let 
us  consider  the  diagram  of  Figure  261  in  which  the  solidification  period  of  steel  is 
shown  as  influenced  by  its  carbon  content.  This  diagram  will  be  discussed  at  greater 
length  in  another  chapter.  Let  us  for  the  present  note  (1)  that  as  the  carbon  increases 
from  0  to  2.0  per  cent  the  solidification  of  the  steel  is  lowered  from  A  to  B,  that  is 


s 

•    -* 


V\K.  25!).  —  Burnt  steel.     Carbon  1  .24  per  cent  .  Fiji.  2(i<).  —  Burnt  steel.    Magnified  :«) 

Magnified  20  diameters.    Quenched  at  a  white  diameters.     iStead.' 

heat.      Unotched.      (Osmond.) 

from  1500  deg.  C.  to  1325  cleg.,  (2)  that  while  carbonless  iron  solidifies  at  a  constant 
temperature,  namely  1500  dog.,  as  the  carbon  increases,  the  range  of  temperature 
covered  by  the  solidification  period  increases  likewise,  extending  from  B  to  ('  with 
2  per  cent  carbon,  that  is  from  1325  to  1130  deg.  ('.  ABC  then  represents  the  solidi- 
fication zone  of  steels  of  increasing  carbon  content  and  the  heating  of  the  metal  to 
any  point  within  this  zone,  when  it  is  partly  melted,  will  cause  it  to  burn.  It  follows 
from  this  that  carbonless  iron  and.  very  low  carbon  steel  can  be  heated  to  a  very  high 
temperature  without  burning,  while  the  danger  of  burning  increases  with  the  carbon. 
With  0.50  per  cent  carbon,  for  instance,  the  burning  zone  extends  from  1400  to  1450 
deg.,  with  1.0  per  cent  it  extends  from  1310  to  1400  deg.,  with  1.50  per  cent  carbon 
from  1210  to  1360  deg.  In  short,  as  the  carbon  increases  the  steel  burns  more  read- 
ily (1)  because  its  melting-point  is  lowered  and  (2)  because  its  solidification  zone, 
which  is  also  its  burning  zone,  is  widened.  According  to  the  theory  it  should  not  be 
possible  to  burn  carbonless  iron,  and  indeed  the  author  does  not  know  thai  the  claim 
has  ever  been  made  that  carbonless  iron  could  be  burnt. 


CHAPTER   XV  — THE   ANNEALING    OF   STEEL 


261 


If  ABC  represents  a  burning  zone  into  which  steel  cannot  be  brought  without 
having  its  useful  qualities  destroyed,  we  naturally  ask  why  all  steels  are  not  so  in- 
jured seeing  that  they  must  pass  at  least  once  through  this  zone  in  cooling  from  the 
molten  condition.  The  reason  why  steel  does  not  burn  on  solidifying  and  further 
cooling  is  explained  by  Howe  on  the  ground  that  while  steel  ingots  or  other  castings 
solidify,  much  hydrogen  is  given  out  which  may  mechanically  restrain  the  oxygen 
from  entering  and  also  counteract  it,  preventing  thereby  the  evolution  of  CO  from 
within  and  the  formation  of  oxidized  films,  the  chief  causes  of_burning.  It  is  also 
possible,  Howe  says,  that  the  greater  kneading  which  an  ingot  undergoes  cures  burn- 


/5OO 


o 


Carbon    per    cenf 

Fig.  261.  —  Diagram  depicting  the  burning  temperature  range. 


ing,  while  the  slight  kneading  possible  in  reworking  a  steel  bar  does  not.  This,  how- 
ever, would  not  explain  why  steel  castings,  which  undergo  no  work  at  all,  are  not 
burnt,  unless  it  is  because  they  are  generally  protected  from  atmospheric  oxidation 
by  their  molds  (Howe) . 

Burning  should  not  be  confounded  with  overheating.  Overheated  steel  has  a 
very  coarse  structure  and  fracture,  but  it  can  be  restored  by  heat  treatment  alone,  or 
at  least  by  heating  and  forging,  while  burnt  steel  is  believed  by  many  to  be  incur- 
able. Overheating  results  from  heating  close  to  but  below  AC  (Fig.  261),  generally 
for  a  considerable  length  of  time,  while,  as  explained,  the  temperature  in  burning  is 
carried  above  AC. 

Important  results  recently  obtained  by  Gutowsky  would  place  the  end  of  the 


262  CHAPTER   XV  — THE   ANNEALING   OF   STEEL 

solidification  of  various  carbon  steels  as  indicated  by  the  dotted  line  in  Figure  261. 
If  these  are  the  correct  temperatures  at  which  solidification  is  complete,  it  follows 
that  the  burning  zone  is  wider  than  was  generally  believed  before  the  publication  of 
these  results. 

Stead  rejecting  the  oxidizing  gases  theory  believes  that  the  burning  of  steel 
resulting  from  heating  the  metal  to  incipient  fusion,  causes  the  formation  of  globules 
or  envelopes  rich  in  phosphorus  (hence  fusible)  round  the  crystals  and  that  their 
presence  or  absence  is  a  proof  of  the  steel  having  been  burnt  or  not.  When  they  occur 
they  are  readily  detected  by  etching  the  metal  with  his  cupric  reagent  (Chapter  II). 

The  burnt  steel  in  Fig.  262  has  been  etched  with  this  reagent.     The  white  net- 


Fig.  262.  —  Burnt  steel  etched  with  Stead's  cupric  rea- 
gent showing  globules  and  envelopes  rich  in  phosphorus. 
Magnified  50  diameters.  (Stead.) 


work  and  white  particles  show  the  segregation  of  portions  rich  in  phosphorus.  Stead 
writes : 

"The  question  is  often  asked  of  the  expert,  when  steel  samples  have  broken  up  at 
the  rolls,  whether  the  material  is  naturally  red  short  or  has  been  burnt.  The  sides  of 
cracks  found  in  the  steel,  whether  simply  red  short  or  burnt,  are  always  lined  with 
oxide;  and  the  expert,  in  conducting  his  examination,  must  be  careful  to  select  por- 
tions free  from  cracks  and  to  polish  and  etch  them.  If  they  be  free  from  globular 
specks  rich  in  phosphorus,  it  may  be  concluded  that  the  material  has  not  been  burnt 
and  is  naturally  red  short,  but  if  such  specks  be  present  the  reverse  conclusion  will 
be  justified." 

Crystalline  Growth  of  Austenite  Above  the  Critical  Range. —  Above  its  critical 
range  steel  is  composed  of  polyhedral  crystalline  grains  of  austenite,  which  are  made 
up  of  small  crystals  (probably  octahedra)  similarly  oriented  in  the  same  grain  but 
whose  orientation  changes  from  one  grain  to  the  next.  Indeed  it  is  this  lack  of  uni- 
formity of  the  orientation  of  the  crystalline  matter  building  up  the  grains  that  gives 


CHAPTER  XV  — THE  ANNEALING   OF   STEEL  263 

existence  to  these  grains,  for  if  all  the  small  crystals  of  which  they  are  composed  were 
similarly  oriented,  clearly  there  would  be  but  a  single  allotrimorphic  crystal  or  grain. 
If  steel  be  maintained  for  a  long  time  above  its  critical  range,  the  austenite  grains  of 
which  it  is  composed  show  a  tendency  to  grow  in  size  through  adjoining  grains  assum- 
ing like  crystalline  orientation  and,  therefore,  merging  into  a  single  and  correspond- 
ingly larger  grain.  This  growth  increases  with  the  temperature  and  with  the  duration 
of  the  treatment.  Given  a  sufficiently  long  time  and  sufficiently  high  temperature, 
but  one  grain  must  be  formed.  This,  as  already  seen  in  Chapter  XVI,  is  actually  what 
takes  place  in  meteorites  during  the  cooling  of  which  the  prevailing  conditions  are 
such  as  to  produce  this  uniformity  of  orientation.  It  follows  from  the  above  considera- 
tions that  on  annealing,  if  the  metal  be  kept  a  long  time  above  its  critical  range,  even 
but  slightly  above  it,  a  coarser  austenite  will  be  formed  which  in  turn  implies,  after 
slow  cooling,  a  coarser  pearlitic  or  sorbitic  structure.  In  hypo-eutectoid  steel  the 
grains  of  austenite  will  expel  some  free  ferrite,  and  in  hyper-eutectoid  steel  some  free 
cementite,  before  being  converted  into  pearlite,  but  the  final  pearlite  grains  will 
nevertheless  increase  in  size  with  the  size  of  the  original  austenite  grains.  The  struc- 
ture of  steel  containing  0.50  per  cent  carbon  kept  two  hours  at  1150  deg.  C.  and  cooled 
in  air  has  been  shown  in  Figure  228.  The  very  large  sorbitic  grains  formed  prove 
the  existence  above  the  range  of  equally  large,  or  even  larger,  austenitic  grains.  The 
cooling  through  the  range  was  so  rapid  that  but  a  small  amount  of  free  ferrite  was 
separated,  the  sorbite  grains,  therefore,  representing  nearly  the  exact  size  of  the  auste- 
nite grains. 

In  Figure  263  an  attempt  has  been  made  to  depict  this  relation  between  the 
austenitic  structure  above  the  range  and  the  corresponding  pearlitic  or  sorbitic  struc- 
ture below  the  range.  The  steel  considered  is  supposed  to  be  hypo-eutectoid  and  to 
contain,  after  slow  cooling,  a  large  proportion  of  free  ferrite.  A  is  intended  to  rep- 
resent a  piece  of  this  steel  made  up  of  nine  relatively  small  austenitic  grains  formed 
on  short  exposure  above  the  range.  On  cooling  through  the  range  ferrite  is  liberated 
and  the  residual  austenite  grains  converted  into  as  many  pearlite  grains  as  shown  in 
A'.  The  small  squares  of  the  matrix  surrounding  the  pearlite  grains  represent  as 
many  small  ferrite  grains.  After  a  longer  exposure,  possibly  at  a  higher  temperature, 
the  steel  will  be  made  up  of  larger  austenite  grains,  say  of  four  grains  as  shown  in  B, 
and  these,  on  slow  cooling  through  the  range,  will  be  converted  after  rejection  of  fer- 
rite into  four  pearlite  grains  as  indicated  in  B'.  Theoretically,  at  least,  we  may  assume 
that  the  temperature  above  the  range  may  be  so  high  and  the  exposure  at  that  tem- 
perature so  long  that  but  a  single  austenite  grain  is  formed,  the  entire  mass  having 
assumed  a  uniform  crystalline  orientation  as  shown  in  C.  On  slow  cooling  a  single 
pearlite  grain  would  then  be  formed  surrounded  by  free  ferrite  in  the  form  of  small 
grains  as  depicted  in  C'.  An  exceedingly  slow  cooling  through  and  below  the  range 
would  have  a  tendency  to  cause  the  free  ferrite  to  crystallize  into  larger  grains  and 
eventually  to  form  but  a  single  grain  as  indicated  in  C".1  Finally,  as  will  be  later 
explained,  on  very  long  exposure  to  a  high  temperature  the  cementite  should,  theo- 
retically at  least,  be  converted  into  as  many  graphite  particles  as  there  are  austenite 
grains  and,  therefore,  into  a  single  graphite  particle  in  case  there  is  but  a  single  grain 
of  austenite,  the  steel  then  consisting,  after  slow  cooling,  of  a  kernel  of  graphite  sur- 
rounded by  uniformly  oriented  ferrite  as  shown  in  C'".  As  already  explained,  how- 

1  Such  slow  cooling,  as  previously  explained,  would  also  have  a  tendency  to  cause  the  spheroi- 
dizing  of  cementite. 


264 


CHAPTER   XV  — THE   ANNEALING    OF   STEEL 


CO 
<M 
tab 

E 


CHAPTER   XV  — THE   ANNEALING    OF   STEEL  265 

ever,  this  breaking  up  of  cementite  into  graphite  and  ferrite  does  not  take  place 
unless  a  considerable  amount  of  carbon  is  present,  namely  over  one  per  cent. 

The  conditions  depicted  in  A',  B',  C',  C",  and  C'"  are  conditions  of  equilib- 
rium, according  to  the  phase  rule,  since  but  two  phases  are  present.  It  is  now  be- 
lieved, however,  that  A',  B',  C',  and  C"  represent  metastable  equilibrium  while  C'", 
only,  represents  stable  equilibrium.  The  phase  rule  will  be  considered  in  another 
chapter  when  these  remarks  will  be  made  clear. 

Crystalline  Growth  of  Ferrite  Below  the  Critical  Range.  • — -Stead  in  1898  showed 
that  strained  ferrite,  when  heated  close  to  but  below  the  critical  range  of  the  metal, 
undergoes  a  marked  crystalline  growth  caused  by  adjoining  ferrite  grains  assuming 
the  same  crystalline  orientation  and  therefore  merging  into  a  larger  grain.  It  seemed 
evident  from  Stead's  experiments  that  unless  the  ferrite  be  strained  (through  cold 


Fig.  204.  —  Steel.  Carbon  0.05  per  cent.  Magnified  (5  diameters.  Subjected  to 
Brinell  ball  test  under  pressure  of  6000  kilograms  and  heated  to  650  deg.  C.  for 
seven  hours.  Vertical  section  through  bottom  of  spherical  depression.  (J.  O. 
Connolly  in  the  author's  laboratory.) 

working  or  otherwise)  it  will  not  grow  on  annealing,  and  also  that  the  presence  of  a 
relatively  small  amount  of  carbon  (some  0.15  per  cent  or  more)  effectively  prevents 
the  growth  of  the  ferrite  grains,  apparently  because  of  the  pearlite  particles  standing 
in  their  way  and  preventing  their  merging.  The  author  recently  conducted  in  his 
laboratory  some  experiments,  the  results  of  which  throw  additional  light  upon  this 
crystalline  growth.1 

The  following  extracts  from  a  paper  by  the  author  presented  at  the  sixth  Con- 
gress of  the  International  Association  for  Testing  Materials  describe  briefly  some  of 
the  most  significant  results  obtained. 

In  Figure  264  is  shown  the  slightly  magnified  structure  of  a  steel  containing  0.05 
per  cent  carbon  which  had  been  subjected  to  the  Brinell  ball  test2  under  a  pressure  of 

1  Those  experiments  wore  made  by  J.  O.  Connolly,  at  the  time  a  research  student  at  Harvard 
University,  now  with  the  American  Steel  and  Wire  Co. 

2  In  using  the  Brinell  ball  test  as  a  suitable  means  of  straining  ferrite  in  order  to  study  its 
growth  on  annealing  the  author  followed  Charpy. 


266  CHAPTER  XV  — THE  ANNEALING  OF  STEEL 

6000  kilograms  and  then  annealed  at  650  deg.  C.  for  seven  hours.  The  section  shown 
is  a  vertical  one  passing  through  the  bottom  of  the  spherical  depression  made  by  the 
10-mm.  ball  used.  It  will  be  obvious  that  the  strain  was  most  severe  at  A,  that  is 
at  the  very  bottom  of  the  depression,  and  that  it  decreased  gradually  in  intensity 
from  A  to  D.  The  following  features  of  the  structure  should  be  noted  as  having 
special  significance:  (1)  At  D  where  the  metal  was  but  slightly  if  at  all  strained  no 
crystalline  growth  occurred,  (2)  at  C  where  the  strain  must  have  been  more  severe  a 
sudden  growth  of  maximum  intensity  has  taken  place,  (3)  from  C  to  B  as  the  severity 
of  the  strain  increases  the  crystalline  growth  shows  a  gradual  decrease,  and  (4)  from 
B  to  A,  that  is  with  further  increase  of  the  intensity  of  the  strain,  no  crystalline 
growth  has  taken  place.  These  observations  point  to  the  conclusion  that  ferrite 


Fig.  265.  —  Steel.  Carbon  0.05  per  cent.  Magnified  6 
diameters.  Subjected  to  Brinell  ball  test  under 
pressure  of  3000  kilograms  and  heated  to  650  deg.  C. 
for  seven  hours.  Vertical  section  through  bottom  of 
spherical  depression.  (J.  O.  Connolly  in  the  author's 
laboratory.) 

grains  will  not  grow  on  annealing  below  the  critical  range  unless  they  have  been 
subjected  to  a  certain  stress  creating  a  certain  strain,  and  that  they  will  not  grow  if 
that  stress,  and  therefore  the  resulting  strain,  has  been  exceeded.  In  other  words, 
they  point  to  the  existence  of  a  critical  strain  producing  growth,  strains  of  greater 
or  less  magnitude  being  ineffective.  The  narrow  region  occupied  by  the  critically 
strained  metal  should  also  be  noted  as  well  as  the  very  sharp  line  of  demarcation  be- 
tween the  critically  strained  and  the  under-strained  metal.  The  separation  of  the 
critically  strained  metal  from  the  over-strained  is  not  so  sharp. 

Similar  experiments  were  repeated  many  times  and  like  results  always  obtained. 

A  piece  of  the  same  steel  was  subjected  to  exactly  the  same  treatment,  except  that 
the  ball  pressure  applied  was  3000  kilograms  or  half  the  pressure  applied  to  the  pre- 
vious one.  The  crystalline  growth  resulting  from  the  annealing  of  this  sample  is 
shown  in  Figure  265.  Because  of  the  smaller  stress  applied,  the  critically  strained 
portion  of  the  metal  is  nearer  the  depression.  This  would  naturally  be  expected. 


CHAPTER   XV  — THE   ANNEALING   OF   STEEL  267 

Here,  as  in  the  previous  case,  we  have  three  distinct  regions:  (1)  the  metal  surrounding 
the  depression  and  extending  to  a  certain  distance  which  was  too  severely  strained  to 
grow,  (2)  the  critically  strained  metal  in  the  form  of  a  spherical  shell,  and  (3)  the 
rest  of  the  metal  unstrained  or  too  feebly  strained  for  the  growth  to  take  place. 
Figure  266  is  a  section  through  a  similar  sample,  the  specimen  having  been  ground 
level  with  the  bottom  of  the  depression.  The  occurrence  in  this  section  of  a  ring  show- 
ing crystalline  growth  will  be  readily  understood. 

In  Figure  267  is  shown  the  structure  of  a  bar  of  the  same  steeLwhich  after  having 
been  completely  bent  was  subjected  to  annealing  (seven  hours  at  650  deg.  C.).  A 
piece  of  the  bent  portion  of  this  bar  was  then  cut  and  a  longitudinal  section  through 


Fig.  266.  —  Steel.  Carbon  0.05  per  cent.  Magnified 
6  diameters.  Subjected  to  Brinell  ball  test  under 
pressure  of  oOOO  kilograms  and  heated  to  650  deg. 
C.  for  seven  hours.  Horizontal  section  through 
bottom  of  spherical  depression.  (J.  O.  Connolly  in 
the  author's  laboratory.) 

its  center  prepared  for  microscopical  examination.  It  will  be  obvious  that  the  upper 
part  of  the  bent  portion  of  the  bar  was  subjected  to  severe  tension  and  the  under 
part  to  severe  compression.  Somewhere  between  the  upper  and  lower  parts  a  neutral 
plane  existed  which  was  subjected  neither  to  tension  nor  to  compression,  and  in  the 
vicinity  of  this  plane  the  metal  was  but  slightly  strained.  Moving  in  both  direc- 
tions from  this  neutral  plane,  the  metal  becomes  gradually  more  severely  strained. 
Figure  267  shows  that  (1)  in  the  center  of  the  bar  no  growth  took  place,  the  metal 
being  here  under-strained,  (2)  as  soon  as  the  critically  strained  portion  was  reached 
a  very  abrupt  growth  occurred  of  maximum  intensity,  and  (3)  this  growth  decreased 
gradually  as  the  metal  became  more  severely  strained,  being  very  slight  if  existing 
at  all  where  the  strain  was  maximum,  that  is,  near  the  upper  and  under  parts  of  the 
bend.  The  widening  of  the  central  zone,  free  from  growth  as  the  distance  from  the 


268  CHAPTER  XV  —  THE  ANNEALING  OF  STEEL 

bend  increases,  is  also  consistent,  with  the  existence  of  a  critical  strain,  for  it  is 
evident  that  the  portion  of  unstrained  or  under-strained  metal  increases  with  that  dis- 
tance. Incidentally  this  experiment  shows  that  tension  is  apparently  as  effective  as 
compression  in  producing  crystalline  growth,  there  being  apparently  no  difference  in 
the  size  of  the  ferrite  grains  between  the  upper  and  under  parts  of  the  bar.  The  criti- 
cally strained  portions  occupy  also  nearly  the  same  position  with  regard  to  the  out- 
side surfaces,  that  is,  they  occur  at  the  same  depth.  Their  width  also  appears  to  be 
the  same. 

Bars  of  the  same  steel  were  subjected  to  compression,  shearing,  and  twisting 
stresses  and  the  results  obtained  were  in  every  case  consistent  with  the  existence  of 
a  critical  strain. 

With  a  view  of  securing  some  data  in  regard  to  the  magnitude  of  the  critical  stress 
needed  to  induce  growth  on  annealing,  a  number  of  bars  of  the  same  steel  were  sub- 


Fig.  267.  —  Steel.  Carbon  0.05  per  cent.  Magnified  3  diameters.  Bar  hent 
double  and  heated  to  650  deg.  C.  for  seven  hours.  Longitudinal  section 
through  center  of  hent  portion.  (J.  O.  Connolly  in  the  author's  laboratory.) 

jected  to  tensile  stresses  of  increasing  intensity,  annealed  at  650  deg.  seven  hours,  and 
in  every  case  a  cross  section  of  the  strained  and  annealed  bar  was  prepared  for  micro- 
scopical examination.  The  elastic  limit  or  rather  yield  point  of  the  metal  was  in  the 
vicinity  of  28,000  Ibs.  per  square  inch,  and  its  ultimate  strength  45,600  Ibs.  per  square 
inch. 

In  Figures  268  to  271  are  shown  the  structures  of  the  bars  subjected  respectively 
to  stresses  of  38,000,  40,000,  42,000,  and  44,000  Ibs.  per  square  inch. 

These  tensile  tests  afford  another  conclusive  evidence  of  the  existence  of  a  critical 
strain  and  the  fact  that  a  stress  of  40,000  Ibs.  per  square  inch  produces  a  marked 
growth  while  stresses  but  slightly  inferior  or  superior,  namely,  38,000  and  42,000  Ibs. 
per  square  inch,  do  not  induce  growth  is  an  indication  of  the  narrowness  of  the  range 
of  the  critical  stress. 

A  striking  illustration  of  the  existence  of  critically  strained  metal  is  shown  in 
Figure  272  after  C.  Chappell,  in  the  form  of  longitudinal  sections  through  broken 
tensile  test  pieces  heated  for  two  hours  at  600  deg.  (\  Here  again  the  demarcation 


CHAPTER  XV— THE  ANNEALING  OF  STEEL 


269 


between  the  under-strained  portion  and  the  critically  strained  one  is  sharper  than 
between  the  latter  and  the  over-strained  portion. 

The  contention  of  some  that  if  time  be  given  the  over-strained  portion  also  will 
grow  does  not  remove  the  gap  existing  between  critically  strained  metal  responding 


l-'ii;.  2f  IS.    -  Tensile  st  ress,  :«,()()(>  Ibs.  per  sc|.  in.         Fig.  269.  —  Tensile  stress,  42,000  Ibs.  per  sq.  in. 


Fig.  270.  —  Tensile  stress,  40.000  Ibs. 


per  sq.  in.        Fig.  271.  —  Tensile  stress,  44,000  Ibs.  per.  sq.  in. 


so  readily  to  the  annealing  treatment  and  over-strained  metal,  if  not  absolutely  re- 
calcitrant, at  least  extremely  refractory  to  the  treatment. 

In  subjecting  electrolytic  iron  of  great  purity  to  the  treatments  producing  the 
crystalline  growth  of  ferrite  described  above,  Stead  reports  that  he  was  unable  to 


270 


CHAPTER   XV  —  THE   ANNEALING   OF   STEEL 


cause  its  grain  to  grow.  The  author  likewise  has  never  succeeded  in  producing  a 
growth,  at  least  in  any  appreciable  degree,  in  wrought  iron  nor  in  practically  carbon- 
less iron  such  as  American  ingot  iron.  It  seems  certain  that  a  small  amount  of  car- 
bon, preferably  in  the  vicinity  of  0.05  per  cent,  must  be  present.  On  the  other  hand 


-21 


-20 


-19 


-18 


-17 


-16 


-15 


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-15 


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he  has  never  succeeded  in  coarsening  appreciably,  by  similar  treatments,  the  grain 
of  steel  containing  over  0.12  per  cent  carbon.  It  would  seem  therefore  that  we  must 
also  recognize  the  existence  of  a  critical  carbon  content  (0.04  to  0.12  per  cent)  above 
or  below  which  ferrite  grains  will  not  grow  on  annealing  below  the  range,  even  if  they 
have  been  critically  strained. 

From  what  has  been  said  of  the  crystalline  growth  of  strained  ferrite  under  its 


CHAPTER   XV  — THE   ANNEALING   OF   STEEL  271 

critical  range,  and  considering  that  cold  drawn  steel  wires  are  frequently  annealed 
at  such  temperature,  it  may  reasonably  be  asked  why  they  are  not  rendered  thereby 
coarsely  crystalline  and  brittle.  Obviously  because  steel  wires  when  subjected  to 
the  annealing  treatment  are  generally  in  a  decidedly  over-strained  condition.  If  very 
low  carbon  steel  wire  be  given  a  draft  so  light  that  it  is  critically  strained,  heating  it 
for  a  sufficient  length  of  time  at  some  600  deg.  C.  will  cause  the  crystalline  growth  of 
ferrite  as  should  be  expected.  In  such  critically  strained  metal,  however,  the  growth 
may  be  prevented  by  heating  to  a  decidedly  lower  temperature,  as  for  instance  to 
525  or  550  deg.  C.,  which  is  sufficient  to  remove  materially  the  brittleness  result- 
ing from  cold  working  or,  of  course,  by  annealing  above  the  critical  range. 

According  to  Kenneth  B.  Lewis,  in  wire  drawing  the  reduction  should  not  be  less 
than  20  per  cent  in  order  to  avoid  the  production  of  critically  strained  metal  and  hence 
dangerous  crystalline  growth  on  subsequent  annealing  below  the  thermal  critical 
range. 

Robin  believes  that  the  straining  of  ferrite  results  in  the  creation  of  a  number  of 
nuclei  (germes)  from  which  new  grains  will  grow  on  annealing.  To  account  for  the 
existence  of  a  critical  strain  it  might  be  argued  (1)  that  in  under-strained  metal  these 
nuclei  are  not  formed,  (2)  that  in  critically  strained  metals  a  few  nuclei  are  produced 
giving  rise  on  annealing  to  a  few  large  grains,  and  (3)  that  in  over-strained  metal  a 
great  many  nuclei  form,  producing  on  annealing  a  great  many  and  therefore,  neces- 
sarily, smaller  grains. 

Beilby's  amorphous  theory  briefly  explained  in  Chapter  V  seems  to  afford  the 
most  acceptable  explanation  of  the  crystalline  growth  of  strained  ferrite  below  its 
critical  range,  on  the  ground  that  the  amorphous  iron  produced  by  straining,  re- 
crystallizes  below  the  range.  Howe  has  suggested  the  following  ingenious  hypoth- 
esis to  account  for  the  existence  of  a  critical  strain:  (1)  the  crystalline  growth  of 
ferrite  is  an  electrolytic  process  in  which  the  amorphous  iron  acts  as  electrolyte, 
(2)  if  the  iron  be  over-strained  the  quantity  of  amorphous  iron  is  so  great,  that  is 
there  is  so  much  electrolyte,  that  electrolysis  can  take  place  across  it  only  very  slowly, 
hence  the  slowness  or  entire  absence  of  crystalline  growth,  (3)  in  under-strained  iron 
there  is  no  growth  because  of  the  presence  of  an  insufficient  amount  of  amorphous 
iron,  that  is  of  electrolyte,  and  (4)  in  critically  strained  metal  the  proportion  of  amor- 
phous iron  is  such  as  to  lead  to  rapid  electrolysis,  that  is  to  rapid  growth.  "In  short," 
Howe  writes,  "the  very  rapid  coarsening  implies  the  presence  of  enough  electrolyte 
to  give  electrolysis  a  chance,  yet  not  so  much  electrolyte  as  to  cause  too  great  a  bar- 
rier to  electrolysis  by  its  width.  Again,  if  the  quantity  of  the  electrolyte  was  exces- 
sive that  would  mean  that  the  quantity  of  crystalline  iron  remaining  was  small. 
Now  it  is  only  the  crystalline  iron  which  can  form  grains  and  hence  can  coarsen,  and 
if  this  iron  is  reduced  to  too  small  a  quantity  it  might  readily  follow  that  the  residual 
crystalline  iron  was  not  abundant  enough  to  form  large  grains." 

Brittleness  of  Low  Carbon  Steel.  —  The  crystalline  growths  and  other  structural 
changes  described  in  the  foregoing  pages  lead  naturally  to  the  consideration  of  the 
brittleness  occasionally  exhibited  by  low  carbon  steel.  Since  steel  containing  very 
little  carbon  is  essentially  made  up  of  ferrite,  its  occasional  brittleness  must  be  due  to 
the  occasional  brittleness  of  ferrite,  a  constituent  by  nature  soft  and  ductile.  Stead 
has  indicated  two  kinds  of  brittleness  from  which  ferrite  may,  and  occasionally  does 
suffer,  namely,  (1)  "inter-granular"  brittleness  and  (2)  "inter-crystalline"  or  "cleav- 
age" brittleness. 


272 


CHAPTER  XV  — THE  ANNEALING  OF  STEEL 


By  inter-granular  brittleness  is  meant  a  lack  of  cohesion  between  the  ferrite  grains 
leading  to  ready  fracture  under  shock,  the  line  of  fracture  following  the  boundary 
lines  of  the  grains.  Such  brittleness  is  usually  due  to  the  presence  of  impurities  form- 
ing brittle  and  more  or  less  continuous  membranes  surrounding  the  grains.  The 
presence  of  much  phosphorus,  however,  appears  to  produce  inter-granular  brittleness 
without  producing  surrounding  membranes. 

Inter-crystalline  or  cleavage  brittleness  is  caused  by  the  ferrite  grains  assuming 
nearly  the  same  crystalline  orientation  so  that  the  plane  of  fracture  follows  the  cleav- 
age planes  and  passes  from  grain  to  grain  almost  in  a  straight  line.  The  diagram 
shown  in  Figure  273  will  make  this  clear.  The  cross  lines  represent  the  cleavage 
planes  in  each  grain.  In  B  the  metal  is  made  up  of  large  ferrite  grains  but  the  crys- 
talline orientation  of  these  grains  is  so  heterogeneous  that  a  line  of  fracture  cannot 


A  B 

l-'ig.  273.  —  (Stead). 


readily  be  developed  and  pass  from  grain  to  grain,  the  abrupt  change  of  crystalline 
orientation  encountered  at  each  boundary  line  acting  as  an  effective  obstruction  to 
its  advance.  In  A,  on  the  contrary,  while  the  grains  are  smaller  they  have  nearly 
the  same  orientation,  hence  fracture  may  proceed  from  grain  to  grain  with  much 
greater  ease  and  in  a  nearly  straight  line.  Fortunately,  so  uniform  a  crystalline  orien- 
tation is  not  often  met  with  and  cleavage  brittleness  is  a  rather  rare  occurrence.  It 
can  be  cured  by  reheating  the  steel  to  900  deg.  or  higher. 

Stead  also  noticed  that  low  carbon  steel  plates  rolled  below  t  he  critical  range  and, 
therefore,  strained,  and  annealed  likewise  below  the  range,  often  exhibit  a  tendency 
to  break  in  three  directions,  namely,  at  45  deg.  to  the  direction  of  rolling  and  at  right 
angles  with  the  surface  of  the  plates,  that  is,  in  the  directions  of  the  three  cleavage 
planes  of  a  cube  having  four  faces  at  45  deg.  to  the  edges,  and  two  faces  parallel 
to  the  surface  of  the  plates.  This  he  calls  "rectangular"  brittleness.  "We  are  led 
from  this  to  conclude,"  Stead  writes,  "that,  just  as  light  impresses  a  latent  image  on 
a  bromide  photographic  plate  which  cannot  be  seen  but  is  developed  and  made  mani- 


CHAPTER   XV  — THE   ANNEALING   OF   STEEL  273 

fest  by  the  action  of  certain  chemical  agencies,  so  the  rolling  appears  to  impress  a 
latent  disposition  in  the  steel  to  crystallize  in  certain  fixed  positions,  and  annealing 
develops  it  afterwards."  The  brittleriess  here  referred  to  is  undoubtedly  caused  by 
the  crystalline  growth  of  strained  ferrite  when  annealed  below  its  critical  range,  as 
fully  explained  in  this  chapter,  the  formation  of  large  ferrite  grains  naturally  causing 
brittleness.  This  kind  of  brittleness  is  sometimes  called  "Stead's  brittleness."  No 
very  satisfactory  explanation  has  so  far  been  offered  to  account  for  this  greater  brit- 
tleness in  certain  directions.  It  may  be  that  the  large  crystalline  grains  of  ferrite 
produced  have  nearly  the  same  orientation  and  that  they  are  so  oriented  as  to  lead 
to  easy  inter-crystalline  rupture  in  the  directions  indicated. 

Conclusions  Regarding  the  Annealing  of  Steel.  —  From  the  foregoing  considera- 
tions it  appears  that  (1)  in  annealing  for  softness  and  ductility  steel  should  be  heated 
slightly  above  its  critical  range  (Ac3  or  Ac3.2  for  hypo-eutectoid  steel,  Ac3.2.i  for  eutec- 
toid  and  hyper-eutectoid  steel)  and  cooled  slowly,  as  for  instance  with  the  furnace 
in  which  it  was  heated,  or  for  greater  strength  in  air,  (2)  in  annealing  for  strength 
and  high  elastic  limit  combined  with  fair  ductility  as  well  as  for  resistance  to  wear, 
to  shock,  and  to  fatigue,  steel  should  be  heated  to  slightly  above  its  critical  range 
followed  by  cooling  in  water,  or  in  oil  according  to  carbon  content  and  requirements 
and  reheated  to  some  500  to  650  deg.  C.  in  accordance  with  the  physical  properties 
desired,  the  lower  temperature  yielding  the  greater  strength  but  the  less  ductility, 
(3)  in  annealing  cold  drawn  wire  and  other  cold  worked  objects,  especially  of  hypo- 
eutectoid  steel,  reheating  close  to  but  below  the  critical  range  (550  to  650  deg.)  is 
generally  sufficient  for  the  aim  in  view,  unless  in  the  case  of  very  low  carbon  steel  so 
strained  as  to  lead  to  ferrite  growth  when  the  temperature  should  be  kept  below  550 
deg.  or  carried  above  the  critical  range,  (4)  in  annealing  castings  they  should  (a)  be 
kept  several  hours  slightly  above  their  critical  range,  or  (b)  be  heated  to  a  tempera- 
ture considerably  above  the  range  followed  by  a  reheating  of  short  duration  slightly 
above  the  range,  or  (c)  be  subjected  to  the  double  treatment  applied  to  forgings,  and 
(5)  by  subjecting  hyper-eutectoid  steel  to  the  spheroidizing  treatment  its  softness 
and  ductility  can  be  considerably  increased  as  well  as  its  resistance  to  wear. 


CHAPTER   XVI 

THE  HARDENING   OF   STEEL 

References  have  already  been  made  in  these  chapters  to  the  invaluable  property 
possessed  by  iron,  containing  a  sufficient  amount  of  carbon,  of  becoming  extremely 
hard  when  suddenly  cooled  from  a  high  temperature  as,  for  instance,  by  quenching 
in  water  from  a  bright  red  heat.  This  operation  is  known  as  the  hardening  of  steel. 
The  close  relation  existing  between  the  hardening  of  steel  and  its  critical  range,  which 
has  also  been  alluded  to,  provides  the  key  to  the  rationale  of  the  hardening  opera- 
tion. This  operation  consists  of  two  distinct  steps  (1)  heating  to  the  hardening  tem- 
perature and  (2)  cooling  from  that  temperature. 

Heating  for  Hardening.  —  In  order  to  harden  steel  it  is  necessary  first  to  heat 
it  above  its  critical  range,  because  it  is  in  passing  through  that  range  that  it  acquires 
hardening  power.  Any  attempt  at  hardening  it  by  cooling  it  suddenly  from  a  tem- 
perature inferior  to  its  critical  range  would  result  in  but  a  very  slight,  if  any,  increase 
of  hardness.  It  is  evident,  therefore,  that  to  possess  hardening  power  steel  must  be 
in  the  condition  of  a  solid  solution  since  the  aggregate  of  ferrite  and  cementite  formed 
on  slow  cooling  through  the  critical  range  cannot  be  hardened  by  sudden  cooling. 
The  metal  should  not  be  heated  much  above  the  top  of  its  range,  because  in  so  doing 
we  coarsen  its  structure  as  explained  in  previous  chapters,  while  we  do  not  increase, 
materially  at  least,  its  hardening  power,  and  our  aim  in  hardening  should  be  to  secure 
maximum  hardness  and  finest  possible  structure.  Quenching  from  a  temperature 
greatly  exceeding  the  critical  range,  moreover,  increases  the  danger  of  warping  and 
cracking  the  objects  in  the  quenching  bath.  Nor  should  the  steel  be  heated  to  a 
temperature  much  above  its  critical  range  and  then  cooled  to  that  range  'before 
quenching,  as  sometimes  recommended,  because  its  structure  is  then  likewise  coars- 
ened by  the  heating  and  slow  cooling  preceding  the  quenching.  Clearly  the  rationale 
of  the  hardening  operation  consists  in  heating  the  metal  just  through  its  critical  range, 
thus  conferring  to  it  both  full  hardening  power  and  finest  possible  structure,  and 
then  in  cooling  it  suddenly  as  soon  as  it  emerges  from  its  range,  lest  its  structure  be 
coarsened  by  heating  above  the  range  or  by  prolonged  exposure  at  the  quenching 
temperature.  This  judicious  method  of  conducting  the  hardening  operation  is  some- 
times described  as  "hardening  on  a  rising  temperature."  Let  it  be  borne  in  mind 
that,  since  the  position  and  width  of  the  critical  range  vary  in  different  steels,  the 
most  desirable  quenching  temperature  will  vary  likewise.  Low  and  medium  high 
carbon  steels  should  be  quenched  at  higher  temperatures  than  high  carbon  steel, 
for  in  order  to  acquire  full  hardening  power  they  should  be  heated  past  their  upper 
critical  points,  namely,  Ac3  or  Ac3.2,  as  the  case  may  be. 

In  practice  the  quenching  temperature  should  be  some  20  to  50  deg.  C.  above 
Ac!  in  hardening  eutectoid  and  hyper-eutectoid  steels  or  above  Ac3.2  in  hardening 

274 


CHAPTER  XVI  — THE  HARDENING  OF  STEEL  275 

hypo-eutectoid  steel.  The  proper  temperatures  to  which  carbon  steels  should  be 
heated  for  hardening  purposes  and  which  are  also  those  generally  suitable  for  anneal- 
ing the  same  steels  have  been  shown  graphically  in  Figure  219,  Chapter  XV. 

Heating  for  hardening  should  be  slow  enough  to  permit  the  steel  to  acquire  grad- 
ually and  evenly  a  uniform  temperature  throughout,  and  should  not  be  too  sudden 
as  for  instance  by  placing  cold  steel  in  red  hot  furnaces. 

Generally  speaking  large  pieces  should  be  heated  to  somewhat  higher  tempera- 
tures than  small  pieces  for  hardening  purposes  it  having  been  found  (Benedicks  and 
Mathews)  that  a  higher  temperature  produces  a  quicker  rate  of  cooling  in  the  quench- 
ing bath.  Mathews  for  instance  states  that  a  3/ie-in.  round  bar  of  tool  steel  will  harden 
at  750  deg.  C.  while  a  M-in.  round  liar  of  the  same  steel  should  be  heated  to  780 
deg.  C. 

The  use  of  lead  or  salt  baths  (chlorides  and  nitrates  of  sodium,  calcium,  potas- 
sium, and  barium)  for  heating  steel  to  the  desired  hardening  temperature  is  to  be 
commended  since  it  promotes  uniformity  of  temperature  throughout  the  pieces  while 
a  suitably  selected  salt  bath  can  readily  be  heated  to,  and  maintained  at,  any  desired 
temperature. 

Cooling  for  Hardening.  —  To  harden  the  steel  the  metal  should  be  cooled  very 
quickly  from  the  temperatures  mentioned  in  the  above  paragraphs  through  its  critical 
range,  generally  by  immersing  it  in  a  medium  capable  of  rapidly  abstracting  heat 
from  it.  The  increase  of  hardness  will  be  the  greater  the  higher  the  carbon  content, 
at  least  up  to  the  eutectoid  point,  and  the  more  rapid  the  cooling,  the  latter,  in  turn, 
depending  upon  the  size  of  the  object  hardened  and  the  nature  of  the  quenching 
bath,  i.e.  its  power  of  abstracting  heat  from  the  cooling  mass.  It  was  long  thought 
that  this  so  to  speak  cooling  power  of  the  bath  depended  chiefly,  if  not  solely,  upon 
its  temperature  at  the  time  of  immersion  and  upon  its  heat  conductivity.  It  was  be- 
lieved, for  instance,  that  mercury  was  a  more  effective  cooling  medium  than  water 
because  of  its  greater  conductivity  for  heat,  that  cold  water  was  more  effective  than 
tepid  water  because  of  its  lower  temperature,  etc.  Recent  investigations  appear 
to  show,  however,  that  the  cooling  power  of  a  quenching  bath  is,  within  limits,  quite 
independent  of  its  actual  temperature  and  of  its  heat  conductivity,  and  even  of  its 
specific  heat.  Benedicks  contends  that  it  depends  almost  exclusively  upon  its  latent 
heat  of  volatilization.  Its  temperature,  however,  should  be  low  enough  to  prevent 
the  adherence  of  vapor  bubbles  to  the  metal.  In  accordance  with  these  views  mer- 
cury, in  Benedicks'  opinion,  is  inferior  to  water  while  saline  solutions  are  not  superior 
to  it.  Methyl  alcohol,  on  the  contrary,  is  a  more  effective  cooling  medium  for  hard- 
ening than  water.  According  to  Le  Chatelier,  also,  mercury  is  less  effective  than 
water  but  in  his  opinion  because  of  its  lower  specific  heat.  Le  Chatelier  believes 
that  the  specific  heat  of  the  liquid  is  the  most  important  factor  influencing  its  value 
as  a  cooling  medium,  its  conductivity  being  of  secondary  importance,  the  loss  of  heat 
taking  place  more  through  circulation  than  through  conductivity.  On  the  other 
hand  Benedicks  contends  that  the  rate  of  flow  has  very  little  influence. 

According  to  Mathews  (1)  the  rate  of  cooling  in  water  quenching  remains  quite 
constant  up  to  a  water  temperature  of  37  deg.  C.,  (2)  brine  solutions  not  only  pro- 
duce a  quicker  rate  of  cooling  but  retain  their  cooling  power  practically  unimpaired 
so  long  as  their  temperature  remains  below  65  deg.  C.,  (3)  while  oil  baths  produce 
a  slower  rate  of  cooling  they  can  be  heated  to  considerably  higher  temperatures  than 
water  or  brine  before  having  their  cooling  power  seriously  diminished. 


276 


CHAPTER   XVI  — THE   HARDENING   OF   STEEL 


Structural  Changes  on  Hardening.  —  Bearing  in  mind  the  enormous  difference 
between  the  properties  of  hardened  steel  and  those  of  the  same  metal  unhardened, 
we  should  naturally  expect  to  find  the  structure  of  hardened  steel  likewise  totally 
different  from  that  of  unhardened  steel.  And  so  indeed  it  is  as  shown  in  Figure  274, 
in  the  case  of  hardened  steel  containing  some  0.50  per  cent  carbon.  To  account  for 
this  structure  let  us  remember  that,  initially,  this  steel  consisted  of  an  aggregate  of 
ferrite  and  cementite  which,  upon  being  heated  through  its  critical  range,  was  con- 
verted into  a  solid  solution  (austenite)  of  carbon  or  carbide  in  gamma  iron.  This 
was  necessary  to  impart  hardening  power.  Had  the  metal  been  allowed  to  cool 
slowly  through  its  critical  range  it  would  have  been  converted  back  into  a  mixture 
of  ferrite  and  cementite.  On  rapid  cooling,  however,  this  transformation  was  pre- 
vented, at  least  in  part,  the  time  necessary  for  its  completion  having  been  denied. 


Fig.  274.  —  Steel.  Carbon  0.45  per  cent .  Mag- 
nified 1000  diameters.  Heated  1<>  S25  deg.  C. 
and  quenched  at  720  deg.  (Osmond.) 


Fig.  275.  —  Steel.  Carbon  1.57  per  cent.  Mag- 
nified 1000  diameters.  Heated  to  1050  dpg.  C. 
and  quenched  in  ire-water.  (Osmond.) 


A  conclusive  evidence  that  the  transformation  does  not  occur  in  its  entirety  is  af- 
forded by  the  absence  of  a  marked  critical  range  on  quick  cooling.  If  the  trans- 
formation of  the  solid  solution  could  be  effectively  prevented  austenite  should  be  the 
constituent  of  hardened  steel.  In  the  commercial  hardening  of  steel,  however,  the 
cooling  is  not  sudden  enough  to  prevent  at  least  a  partial  transformation  of  austenite, 
not  into  ferrite  and  cementite  but  into  a  more  or  less  transitory  form,  marking  t  In- 
first  step  of  that  transformation  and  called  "martensite."  Very  frequently  the  rate 
of  cooling  is  not  sufficiently  rapid  to  prevent  the  martensite  from  further  partial 
transformation  into  a  second  transition  constituent  known  as  "troostite."  Marten- 
site  and  troostite,  then,  are  the  ordinary  constituents  of  commercially  hardened  steel. 
It  will  now  be  profitable  to  consider  at  some  length  the  occurrence,  nature,  and  prop- 
erties of  the  three  constituents  chiefly  concerned  in  dealing  with  hardened  steel, 
namely,  austenite,  martensite,  and  trooslilc. 


CHAPTER   XVI  — THE   HARDENING   OF   STEEL 


277 


AUSTENITE 

Nature  of  Austenite.1  —  Austenite  is  universally  considered  as  a  solid  solution 
of  carbon  or,  more  probably,  of  the  carbide  Fe3C  in  gamma  iron.2  All  steels  above 
their  critical  range  are  made  up  of  this  solid  solution.  It  follows  that  the  carbon  con- 
tent of  austenite  varies,  like  that  of  steel,  from  a  mere  trace  to  some  1.75  or  2  per 
cent.  It  is  not,  therefore,  a  constituent  of  constant  composition. 

Occurrence  of  Austenite.  —  While  present  in  all  steels  above,  their  critical  range 
austenite  is  very  rarely  found  in  ordinary  steels  cooled  to  atmospheric  temperature. 
This  is  due  to  the  rapidity  with  which  austenite  is  transformed  on  cooling  through 
the  critical  range  if  not  into  an  aggregate  of  ferrite  and  cementite,  at  least  into  some 


Fig.  276.  —  Steel.    Carbon  1.57  per  cent.    Magnification  not  stated.    Heated  to 
1050  deg.  C.  and  quenched  in  ice-water.     (Osmond.) 


transition  stages.  In  the  commercial  hardening  of  ordinary  carbon  steel  the  pas- 
sage of  the  metal  through  its  range  is  never  sufficiently  rapid  to  retain  in  the  cold  a 
small  amount  even  of  undecomposed  austenite.  To  prevent  the  transformation  of 
a  portion  of  the  austenite  the  conditions  generally  affecting  the  hardening  of  the 
metal  must  be,  so  to  speak,  greatly  intensified:  (1)  the  steel  should  be  highly  car- 
burized,  (2)  quenching  should  be  from  a  high  temperature  (1000  deg.  C.  or  more), 
and  (3)  a  very  effective  quenching  bath  should  be  used  such  as  ice-cold  water.  In 
Figure  275  is  shown,  after  Osmond,  the  structure  of  steel  containing  1.57  per  cent 

1  This  name  was  suggested  by  Osmond  in  honor  of  the  late  Sir  William  Roberts-Austen.    Aus- 
tenite has  also  been  called  mixed  crystals  and  gamma  iron  and  by  some  writers,  wrongly,  martensite. 

2  Arnold  believes  that  austenite  is  the  carbide  Fe^C  (hardenitc)  holding  in  solution  ferrite  in 
hypo-eutectoid  steel  and  cementite  in  hypor-eutectoid  steel. 


278  CHAPTER   XVI  —  THE   HARDENING   OF   STEEL 

carbon  heated  to  1050  deg.  C.  and  quenched  in  ice-water.  The  magnification  is 
1000  diameters.  The  dark-colored,  zigzag  constituent  is  martensite;  the  light  matrix, 
or  background,  is  austenite.  The  structure  of  the  same  steel,  under  lower  magnifica- 
tion, is  seen  in  Figure  276.  By  the  drastic  quenching  treatment  just  described  it  is 
possible,  in  the  case  of  high  carbon  steel,  to  retain  more  than  one  half  of  the  steel  in 
its  austenitic  condition. 

The  retention  of  austenite  in  the  cold  is  greatly  helped  by  the  presence  of  some 
elements  such  as  manganese  and  nickel  which  lower  the  position  of  the  transforma- 
tion range,  eventually  depressing  it  below  atmospheric  temperature  and,  therefore, 


Fig.  277.  —  Steel.  Carbon  l.CO  per  cent,  manganese  1.00  per 
cent.  Magnified  300  diameters.  Heated  to  1400  deg.  C. 
and  quenched  in  ice-cold  water.  (Robin.) 


causing  the  steel  to  remain  austenitic  even  after  slow  cooling.  This  actually  takes 
place  in  the  presence  of  some  12  per  cent  manganese  or  25  per  cent  nickel.  Robin, 
by  quenching  in  ice-cold  water  from  a  temperature  of  1400  deg.  C.  a  very  small 
piece  of  steel  (1  to  2  cubic  centimeters)  containing  1.60  per  cent  carbon  and  1  per 
cent  manganese,  was  able  to  retain  it  in  an  austenitic  condition  (Fig.  277).  Mau- 
rer,  likewise,  succeeded  in  retaining  in  its  austenitic  condition  a  steel  containing  2 
per  cent  manganese  and  2  per  cent  carbon  by  quenching  it  in  ice-cold  water  from 
a  temperature  of  1100  deg.  (Fig.  278).  As  the  manganese  increases  the  retention  of 
austenite  becomes  easier,  that  is,  the  quenching  need  not  be  so  drastic  nor  the  carbon 
content  so  high.  Finally  with  10  or  more  per  cent  of  manganese  and  one  or  more 
per  cent  carbon  the  steel  remains  austenite  after  slow  cooling.  The  structure  and 
properties  of  manganese  steel  will  be  considered  in  another  chapter. 

To  sum  up:  (1)  austenite  is  never  produced  in  the  commercial  hardening  of  or- 
dinary carbon  steel;  (2)  it  may  be  retained  in  the  cold,  however,  associated  with 


CHAPTER  XVI  — THE  HARDENING  OF  STEEL 


279 


considerable  martensite  in  quenching  very  high  carbon  steel,  from  a  very  high  tem- 
perature in  ice-cold  water  as,  for  instance,  by  quenching  steel  containing  not  less 
than  1.50  per  cent  carbon  from  1000  deg.  C.  or  higher;  (3)  in  the  presence  of  1  per 
cent  of  manganese  very  small  pieces  of  very  high  carbon  steel  may  be  retained  wholly 
in  their  austenitic  condition  by  quenching  them  from  a  very  high  temperature  as, 
for  instance,  by  quenching  in  ice-cold  water  from  1400  deg.  C.  small  pieces  of  steel 
containing  1  per  cent  of  manganese  and  not  less  than  1.5  per  cent  carbon  (Robin); 
(4)  with  increasing  proportions  of  manganese  the  transformation^of  austenite  may 
be  prevented  in  steel  containing  less  carbon  and  quenched  from  lower  temperatures 
(Maurer);  (5)  manganese  steels  containing,  for  instance,  10  or  more  per  cent  manga- 
nese and  one  or  more  per  cent  carbon  remain  austenitic  after  slow  cooling;  (6)  nickel 
steel  containing  some  25  per  cent  of  nickel  likewise  remains  austenitic  on  slow  cooling. 
Benedicks  contends  that  in  the  preservation  of  austenite  in  carbon  steel  by  rapid 
cooling  an  important  part  is  played  by  the  very  great  pressure  to  which  the  metal 


Fig.  278.  —  Steel.     Carbon  1.94  per  cent,  manganese  2.00  per 
cent.    Heated  to  1100  deg.  C.  and  quenched  in  ice-cold  water. 

(Maurer.) 


is  subjected,  (1)  because  of  the  shrinkage  of  the  exterior  portion  or  outer  shell  on 
the  interior  and  (2)  because  of  the  dilatation  accompanying  the  change  from  gamma 
to  beta  iron.  Were  it  not  for  this  pressure  Benedicks  believes  that  the  transforma- 
tion of  austenite  could  not  be  prevented.  As  an  evidence  of  this  he  shows  that  aus- 
tenitic steels  produced  by  quenching  are  austenitic  only  in  their  interior,  i.e.  where 
the  pressure  had  been  greatest,  the  outside  layers  in  which  the  pressure  was  small  or 
nil  being  martensitic.  He  shows,  further,  that  on  removing  by  grinding  the  marten- 
sitic  shell  the  austenitic  core,  in  turn,  becomes  martensitic  owing  to  the  removal  of 
the  pressure  exerted  upon  it  by  that  shell.  Again  the  quenching  of  steel  cylinders 
surrounded  by  cast-iron  shells  resulted  in  the  formation  of  austenite  close  to  the  skin 
of  the  steel  cylinders  owing  apparently  to  the  very  great  pressure  exerted  upon  the 
steel  by  the  contraction  of  the  iron  shells. 

Etching  of  Austenite.  -  -  The  etching  reagents  usually  applied  to  bring  out  the 
structure  of  unhardened  steel,  namely,  picric  acid,  nitric  acid,  tincture  of  iodine,  etc., 
do  not  always  yield  satisfactory  results  in  the  case  of  hardened  steel.  Kourbatoff 
discovered  a  complex  reagent  which  often  produces  greater  contrasts  between  the 


280  CHAPTER   XVI  — THE   HARDENING   OF   STEEL 

various  constituents.  It  is  made  up  by  mixing  one  part  of  amyl  alcohol,  one  part  of 
ethyl  alcohol,  one  part  of  methyl  alcohol,  and  one  part  of  a  4  per  cent  solution  of 
nitric  acid  in  acetic  anhydride  and  should  be  prepared  just  before  use. 

Heyn  recommends  for  etching  hardened  steel  a  solution  containing  one  part  of 
hydrochloric  acid  and  99  parts  of  absolute  alcohol.  More  uniform  results  are  ob- 
tained if  a  weak  current  of  electricity  be  passed  through  the  solution,  the  samples  to 
be  etched  forming  the  positive  pole  while  the  negative  electrode  may  consist  con- 
veniently of  a  piece  of  sheet  lead.  With  the  assistance  of  the  electric  current  the  use 
of  a  very  dilute  aqueous  solution  is  advisable,  namely,  one  part  of  hydrochloric  acid 
in  500  parts  of  distilled  water. 

Osmond,  likewise,  used  successfully  a  solution  of  10  per  cent  of  hydrochloric  acid 
in  water  by  which  the  martensite  is  colored  darker  than  austenite,  the  treatment  re- 
quiring several  minutes.  Osmond  writes:  "There  is  more  regularity  obtained  by 
having  the  specimen  connected,  by  means  of  a  platinum  wire,  with  the  positive  pole 
of  a  bi-chromate  cell,  a  strip  of  platinum  placed  in  the  acid  being  connected  with  the 
negative  pole.  In  this  way  the  specimen  becomes  the  anode,  and  the  platinum  the 
cathode." 

Benedicks  recommends  for  the  etching  of  martensito-austenitic  steel  a  5  per  cent 
alcoholic  solution  of  metanitrobenzol-sulphonic  acid  which  always  darkens  marten- 
site  more  than  austenite.  Immersions  of  some  fifteen  seconds  are  generally  sufficient. 

Structure  of  Austenite.  —  When  austenite  and  martensite  occur  in  the  same  sam- 
ple the  latter  is  generally  colored  darker  than  the  former  (Figs.  275  and  276) .  Marten- 
site,  moreover,  is  readily  distinguishable  because  of  its  zigzag  or  needle  shape.  Some 
writers  claim  that  martensite  is  sometimes  colored  less  than  austenite.  Indeed  Maurer 
contends  that  this  is  always  so,  arguing  that  if  most  photomicrographs  indicate  the 
contrary  it  is  because  the  martensite  had  undergone  a  certain  amount  of  tempering 
resulting  in  the  formation  of  some  troostite  as  later  explained.  According  to  this 
writer,  in  order  to  prevent  any  tempering  of  the  martensite,  and  therefore  the  forma- 
tion of  dark-colored  troostite,  great  care  must  be  exercised  in  sawing,  polishing,  etc. 
To  this  Benedicks  replies  that  it  cannot  always  be  so  for  in  quenching  austenite  in 
liquid  air  martensite  is  formed  which  must  be  free  from  troostite  and  which,  never- 
theless, is  darker  than  austenite.  It  may  be  asked,  however,  whether  it  is  certain 
that  the  martensite  produced  in  this  way  is  actually  free  from  troostite.  Evidences 
of  a  more  conclusive  nature  are  needed  to  account  satisfactorily  for  the  shifting  in 
the  relative  coloration  of  austenite  and  martensite  when  occurring  side  by  side.  Pure 
austenite  is  made  up  of  polyhedral  grains,1  (see  Figs.  277  and  278)  which,  as  explained 
in  previous  chapters  in  connection  with  the  structure  of  gamma  iron,  are  undoubtedly 
made  up  of  true  crystals,  small  octahedra  according  to  Osmond.  It  should  be  noted 
that  when  austenite  occurs  in  the  presence  of  much  martensite  (Fig.  275)  its  poly- 
hedral structure  is  not  brought  out.  Twinnings  are  frequently  observed  in  austenite 
(see  Chapter  V,  Fig.  124)  although  it  has  been  claimed  that  they  form  only  after 
straining,  especially  if  followed  by  annealing. 

Baykoff  succeeded  in  etching  austenite  above  the  critical  range  of  the  steel,  that 
is,  in  a  range  of  temperature  where  it  is  stable.  He  accomplished  this  by  heating 
polished  steel  samples  in  a  porcelain  tube  through  which  a  current  of  hydrogen  was 

1  Because  of  this  structure  Guillet  and  some  other  writers  refer  to  si  eels  composed  of  austcnite 
as  "polyhedral"  steels.  Tnis  doss  not  ssem  advisable  as  it  may  lead  to  confusion,  for  other  steels 
also  have  polyhedral  structures,  to  wit,  very  low  carbon  (ferritis;  steels. 


CHAPTER   XVI  — THE   HARDENING   OF   STEEL 


281 


kept  circulating  and  by  passing  through  it  gaseous  hydrochloric  acid  when  the  de- 
sired temperature  had  been  obtained.  The  resulting  structures  were  found  to  be 
polyhedral  even  in  the  presence  of  very  little  carbon,  thus  confirming  the  previous 
belief  as  to  the  crystalline  character  of  austenite. 


Carbon 
per  cent 

0.8( 
0.91 
.0! 
.1C 
.20 
.30 
.40 

1.50 
1.55 
1.50 
1.40 
1.30 
1.20 
1.10 
l.OC 
l.Of 
O.S( 
O.TC 
O.fM 
0.50 

0.40 
0.3(1 


HarUc.uitc  and 
Martcnsite 


Austenite  and 
Hardenite 


!  Hardenite  and 
!  Martensite 


Fig.  279.  —  Showing  the  relative  softness 
of  austenite.     (Osmond.) 


Properties  of  Austenite.  —  Since  the  carbon  content  of  austenite  varies  from  a 
mere  trace  to  nearly  2  per  cent  it  may  well  be  expected  that  its  physical  properties 
will  likewise  vary,  i.e.  that  it  will  increase  in  hardness  and  strength  and  decrease  in 
ductility  as  the  carbon  increases.  Osmond  has  shown  conclusively  that  austenite 
was  softer  than  martensite  of  identical  carbon  content.  When  it  is  remembered  that 


282  CHAPTER    XV1--THK    HARDEXIXG   OF   STEEL 

in  order  to  produce  austenite  in  ordinary  carbon  steel  all  the  factors  generally  in- 
creasing the  hardness  of  the  metal  must  be  intensified,  it  is  at  first  surprising  that 
the  energetic  quenching  treatment  required  should  yield  a  softer  metal.  The  con- 
clusion must  be  that  gamma  iron  is  softer  than  beta  iron.  This  relative  softness  of 
austenite  is  well  shown  by  Osmond  in  Figure  279  which  represents  the  structure  of  a 
bar  of  steel  containing  1.55  per  cent  carbon  in  the  center  and  a  gradually  decreasing 
amount  towards  the  outside.  This  bar  was  heated  to  1050  deg.  C.  and  quenched  in 
mercury  at  a  temperature  of  —  9  deg.  C.  After  polishing  but  before  etching  a  needle 
was  repeatedly  drawn  across  it  from  end  to  end  with  even  pressure.  The  photograph 
clearly  shows  that  the  needle  scratched  the  steel  (1)  where  it  contains  so  little  carbon 


Fig.  280.  —  Nickel  austenitic  steel  quenched  in  liquid  air.    Magni- 
fied 250  diameters.     (Osmond.) 


(0.40  to  0.60  per  cent)  that  it  was  only  partly  martensitic  and  hence  relatively  soft, 
(2)  in  those  regions  which  because  of  very  high  carbon  content  (1.30  to  1.55  per  cent) 
were  partly  austenitic,  and  (3)  that  it  failed  to  scratch  it  in  those  regions  which  be- 
cause of  a  more  moderate  amount  of  carbon  (0.70  to  1.20  per  cent)  were  fully  marten- 
sitic and,  therefore,  very  hard.  It  is  also  well  known  that  high  carbon  austenitic 
manganese  steel,  while  extremely  difficult  to  machine,  can  be  readily  scratched  by  a 
needle,  being  mineralogically  softer,  therefore,  than  high  carbon,  martensitic  steel. 
Rosenhain  and  Humfrey  have  shown  that  above  the  critical  range  austenite  (gamma 
iron)  was  much  softer  than  beta  iron.  Since  steel  above  its  critical  range  is  non- 
magnetic we  should  expect  steels  which  remain  austenitic  in  the  cold  to  be  non- 
magnetic. This  we  know  to  be  the  case,  for  manganese  as  well  as  nickel  austenitic 
steels  are  non-magnetic. 

Some  of  the  physical  properties  of  austenite  may  be  inferred  from  the  known 


CHAPTER  XVI  — THE   HARDENING   OF   STEEL  283 

properties  of  austenitic  steels  such  as  manganese  and  high  nickel  steels.  These  are 
known  to  be  very  ductile  (after  suitable  heat  treatment),  tenacious,  of  low  elastic 
limit,  to  possess  very  high  resistance  to  wear  although  their  mineralogical  hardness  is 
not  excessive  and  to  be  machined  only  with  great  difficulty.  They  have,  like  gamma 
iron,  a  very  high  electrical  resistance. 

It  has  already  been  pointed  out  that  the  crystallization  of  austenite  is  probably 
cubic,  the  octahedron  being  its  prevailing  crystalline  form.  Le  Chatelier,  however, 
believes  that  austenite  crystallizes  in  the  orthorhombic  system  with  octahedral 
cleavage.  On  slow  cooling  through  the  critical  range  in  the  absence  of  considerable 
quantities  of  retarding  elements  such  as  manganese  and  nickel,  austenite  rejects  a 
sufficient  amount  of  ferrite  in  hypo-eutectoid,  or  of  cementite  in  hyper-eutectoid, 
steel  to  assume  the  eutectoid  composition  (0.85  per  cent  C.  or  thereabout)  when  it 
is  converted  bodily  into  pearlite.  This  transformation  is  not  sudden,  however, 
several  transition  constituents  being  formed,  namely,  martensite,  troostite,  and  sor- 
bite. 

It  will  be  seen  in  another  chapter  that  on  tempering  austenite,  that  is,  on  reheat- 
ing it  below  the  critical  range  of  the  metal  it  is  likewise  converted  gradually  and  suc- 
cessively into  martensite,  troostite,  and  sorbite  or  according  to  some  writers  directly 
into  troostite  and  then  into  sorbite. 

Quenching  nicke!  austenitic  steel  in  liquid  air  results  in  the  formation  of  marten- 
site  with  increased  volume  causing  swellings  of  the  polished  surface  as  shown  in 
Figure  280. 


MARTENSITE 

Nature  of  Martensite.1  —  It  is  very  generally  believed  that  martensite  corre- 
sponds to  an  early  stage  in  the  transformation  of  austenite  in  passing  through  the 
critical  range.  Opinions  differ,  however,  as  to  its  exact  nature.  Accepting  the  possi- 
bility of  iron  existing  under  three  allotropic  forms,  namely,  as  gamma,  beta,  and  alpha 
iron,  and  the  carbon  under  two  distinct  conditions,  namely,  as  the  crystallized  car- 
bide FesC  or  cement  carbon  and  of  this  carbide  or  possibly  elementary  carbon  being 
dissolved  in  iron,  i.e.  as  hardening  carbon,  what  are  the  probable  conditions  of  these 
two  constituents  in  martensite?  Osmond  and  many  others  believe  that  in  martensite 
iron  is  present  chiefly  in  its  beta  condition,  holding  carbon  in  solution,  hence  the 
great  hardness  of  that  constituent.  Since  martensite  is  magnetic,  however,  it  must 
also  contain  an  appreciable  quantity  of  magnetic  alpha  iron.  This  theory,  which  may 
be  called  the  allotropic  theory,  was  at  one  time  widely  held.  Le  Chatelier,  not  believ- 
ing in  the  existence  of  beta  iron,  considers  martensite  as  essentially  a  solid  solution  of 
carbon  in  alpha  iron,  owing  its  great  hardness  to  its  state  of  solid  solution  and  its 
magnetism  to  the  presence  of  alpha  iron.  Edwards  and  Carpenter  contend  that 
austenite  and  martensite  are  in  reality  the  same  constituent,  namely,  a  solid  solu- 
tion of  carbon  in  gamma  iron,  differing  only  in  structural  aspect,  the  needles  of 
martensite  resulting  from  the  twinning  of  austenite  caused  by  the  severe  pressure 
exerted  upon  it  during  rapid  cooling.  Kroll  also  speaks  of  martensite  as  represent- 
ing the  "mutilated  structure  of  austenite  due  to  twinning."  Arnold  believes  that 

1  This  name  was  selected  by  Osmond  in  honor  of  A.  Martens,  a  distinguished  German  metallur- 
gist and  testing  engineer. 


284  CHAPTER  XVI  — THE  HARDENING  OF  STEEL 

martensite  is,  like  austenite,  the  carbide  Fe24C  holding  in  solution  ferrite  in  hypo- 
eutectoid  steel  and  cementite  in  hyper-eutectoid  steel. 

Careful  consideration  of  the  evidences  at  hand  leads  to  the  adoption  of  the  first 
theory  (Osmond's)  as  the  one  best  supported.  That  martensite  is  to  a  great  extent 
a  solid  solution  seems  evident  from  the  fact  that  it  contains  a  great  deal  of  hardening, 
i.e.  dissolved  carbon,  and  it  seems  probable  that  beta  iron  is  the  solvent,  for  if  gamma 
iron  were  the  solvent  it  would  not  explain  the  greater  hardness  of  martensite  com- 
pared to  that  of  austenite  while  we  have  good  reason  to  doubt  the  power  of  alpha 
iron  to  dissolve  carbon  seeing  that  below  the  critical  range,  i.e.  when  in  its  alpha 
form,  iron  will  not  absorb  carbon. 

Occurrence  of  Martensite.  —  Martensite  is  most  readily  obtained  through  the 
quenching  of  small  pieces  of  high  carbon  steel  in  cold  water;  in  the  case  of  large 


Fig.  281.  —  Steel.  Carbon  1.25  per  cent.  Mag- 
nified 150  diameters.  Heated  to  1232  deg. 
C.  and  quenched  in  oil.  (C.  C.  Buck,  Cor- 
respondence Course  student.) 

pieces,  while  the  outside  portion  may  be  martensitic  their  center  is  likely  to  be  partly 
troostitic.  In  low  carbon  steel  it  is  more  difficult  still  to  prevent  the  formation  of 
some  troostite  while  in  steel  containing  very  little  carbon  free  ferrite  as  well  is  likely 
to  be  present.  In  very  high  carbon  steel  some  free  cementite  is  generally  associated 
with  the  martensite. 

Etching  of  Martensite.  —  Dilute  alcoholic  solutions  of  picric,  nitric,  or  hydro- 
chloric acid  generally  bring  out  satisfactorily  the  structure  of  martensite  but  the 
Kourbatoff  reagent,  already  described,  sometimes  yields  better  results.  Martensite 
generally  darkens  more  quickly  than  austenite  but  always  remains  much  lighter  than 
troostite. 

Structure  of  Martensite.  —  Martensitic  structures  are  shown  in  Figures  274  and 
281.  Osmond  describes  the  structure  of  martensite  as  consisting  of  three  systems  of 
fibers,  respectively  parallel  to  the  three  sides  of  a  triangle  and  crossing  each  other 
frequently.  Osmond  also  states  that  when  the  metal  contains  less  carbon  the  needles 


CHAPTER   XVI  — THE   HARDENING   OF   STEEL  285 

are  longer  and  more  clearly  differentiated,  other  things  being  equal.  According  to 
crystallographers  these  markings,  in  reality  cleavages  of  octahedra,  indicate  crystal- 
lites of  the  cubic  system  and,  therefore,  afford  an  additional  evidence  of  the  cubic 
crystallization  of  austenite  from  which  martensite  is  derived.  Osmond  and  Cartaud 
refer  to  them  as  probable  pseudomorphs  of  twinnings  due  to  tension,  occurring  in 
gamma  iron  through  partial  formation  of  the  bulky  beta  and  alpha  modifications. 

Properties  of  Martensite.  —  The  carbon  content  of  martensite  varies  from  a  mere 
trace  to  as  much  as  one  per  cent,  and  possibly  more,  in  very  suddenly  cooled  hyper- 
eutectoid  steels.  In  high  carbon  steels,  however,  it  is  difficult  to  prevent  the  setting 
free  of  much  of  the  excess  cementite  even  on  very  quick  cooling.  From  this  varia- 
tion of  its  percentage  of  carbon  it  follows  that  the  properties  of  martensite  must  also 
vary.  As  the  carbon  increases  its  hardness  and  strength  increase  while  its  ductility 
decreases,  martensitic  steels  being  generally  hard  and  brittle  and,  therefore,  unforge- 
able  in  the  cold. 

It  will  be  seen  in  another  chapter  that  on  heating  martensite  below  the  critical 
range,  i.e.  on  tempering  it,  it  is  converted  first  into  troostite  and  then  into  sorbite. 


TROOSTITE 

Nature  of  Troostite.1  = —  While  most  writers  believe  that  troostite  represents  a 
condition  of  the  steel  resulting  from  the  transformation  of  martensite  and,  therefore, 
a  further  step  in  the  transformation  of  austenite,  much  difference  of  opinion  exists  as 
to  its  exact  nature.  The  controversy  has  given  rise  to  a  very  large  and  apparently 
exaggerated  amount  of  discussion.  Here,  as  in  the  case  of  martensite,  we  must  con- 
sider the  possibility  of  the  iron  existing  in  the  gamma,  beta,  or  alpha  form  or  in  two 
or  even  all  three  of  these  conditions,  while  the  carbon  may  exist  as  cement  carbon  or 
as  hardening  carbon  or  partly  as  cement  and  partly  as  hardening  carbon.  Then 
the  association  between  iron  and  carbon  may  be  of  the  nature  of  an  aggregate  or  of  a 
solid  solution  or  partly  aggregate  and  partly  solution  or,  indeed,  half  way  between 
aggregate  and  solution,  namely,  resembling  a  colloidal  solution,  an  emulsion,  or  an 
uncoagulated  substance.  Nearly  every  conceivable  hypothesis  has  been  suggested  to 
account  for  the  nature  of  troostite.  It  has  been  described  as  a  solid  solution  of  car- 
bon or  of  carbide  in  gamma  iron,  in  beta  iron,  and  in  alpha  iron.  It  has  also  been 
suggested  that  it  might  be  pure  beta  iron. 

In  later  years,  thanks  chiefly  to  the  enlightening  discussions  of  Benedicks  sup- 
ported by  the  weighty  evidence  of  skilfully  conducted  experiments,  metallographists 
have  come  to  regard  troostite  as  an  uncoagulated  mixture  of  the  constituents  of 
martensite  and  sorbite,  that  is,  of  (1)  carbide  dissolved  in  beta  iron,  (2)  crystallized 
FesC,  and  (3)  crystallized  alpha  iron  —  clearly  martensite  passing  to  sorbite.  Bene- 
dicks compares  it  to  a  colloidal  solution2  while  Arnold  had  previously  described  it  as 

1  The  name  troostite  was  selected  by  Osmond  in  honor  of  the  French  chemist  Troost. 

2  A  colloid  may  be  regarded  as  a  substance  passing  from  the  state  of  solution  to  that  of  an  ag- 
gregate or  vice  versa;  it  is  no  longer  a  solution  but  not  yet  an  aggregate.    To  express  it  more  scien- 
tifically, while  not  a  true  solution  the  particles  of  solvent  and  solute  arc  ultra-microscopic.    Accord- 
ing to  Le  Chatelk-r  so-called  colloidal  solutions  are  in  no  way  solutions,  but  merely  liquids  holding  in 
suspension  very  finely  divided  particles;  the  expression,  he  says,  should  not  be  used. 


286 


CHAPTER  XVI  — THE   HARDENING   OF   STEEL 


Fig.  282.  —  Troostite  in  steel  containing  1.50  per  cent  carbon.     Magnified  150 

diameters. 


Fig.  283.  —  Same  as  Figure  282  but  magnified  800  diameters. 


CHAPTER   XVI  — THE    HAKDKMXC    OF   STEEL 


287 


Fig.  284.  —  Troostite  in  steel  containing  1.50  per  cent  carbon.     Magnified  1.50  diameters. 


Fig.  285.  —  Same  as  Figure  284  but  magnified  500  diameters. 


288 


CHAPTER  XVI  — THE  HARDENING  OF  STEEL 


"emulsified"  pearlite.1  The  existence  of  considerable  dissolved  (hardening)  carbon 
in  troostite  is  proven  by  analysis  as  well  as  the  existence  of  considerable  crystallized 
Fe3C  (cement  carbon).  Its  relatively  great  hardness  points  strongly  to  the  presence 
of  a  considerable  amount  of  beta  iron  while  its  magnetism  demands  the  presence  of 
alpha  iron.  Benedicks'  hypothesis  is  consistent  with  what  we  know  of  the  formation 
of  troostite  and  of  its  properties.  McCance  considers  troostite  to  be  amorphous 
alpha  iron  associated  with  carbon  or  a  carbide,  the  nature  of  the  bond  between  the 
two  not  being  clearly  stated. 


^ 


Fig.  286.  —  Troostite  and  martensito-austenitic  matrix  in  steel 
containing  1.50  per  cent  carbon.  Magnified  150  diameters. 
(Boylston.) 


-  In  the  report  of  the  Committee  on  the  Nomenclature  of  the  Microscopical  Con- 
stituents of  Iron  and  Steel  of  the  International  Association  for  Testing  Materials, 
troostite  is  defined  as  follows:  "probably  aggregate.  In  the  transformation  of  aus- 
tenite,  the  stage  following  martensite  and  preceding  sorbite  ...  An  uncoagulated 
conglomerate  of  the  transition  stages." 

Occurrence  of  Troostite.  —  In  order  to  produce  troostite  on  cooling  steel  from 
above  its  critical  range,  it  is  necessary  that  the  cooling  through  the  range  should 
be  so  regulated  as  to  allow  it  to  form  and  at  the  same  time  prevent  its  further  trans- 
formation (into  sorbite  and  pearlite).  These  conditions  may  prevail  (1)  in  cooling 
slowly  to  the  middle  of  the  range,  thus  permitting  the  formation  of  troostite  (see 

1  "Emulsified  carbide  present  in  an  excessively  fine  stale  of  division  in  tempered  steels."    (1895.) 


CHAPTER   XVI  — THE   HARDENING   OF   STEEL  289 

Fig.  288),  and  then  quickly  to  atmospheric  temperature,  thus  retaining  troostite,  and 
(2)  in  cooling  through  the  range  at  a  rate  uniform  throughout  but  so  regulated  as  to 
cause  the  production  and  retention  of  troostite  (Fig.  288)  as,  for  instance,  quenching 
large  pieces  in  water  when  the  central  portions  at  least  will  be  troostitic,  or  quench- 
ing smaller  pieces  in  oil.  It  will  be  explained  in  the  next  chapter  that  troostite  may 
also  be  produced  by  tempering  (i.e.  reheating  below  the  critical  range)  austenitic 
and  martensitic  steels. 

Troostite  is  readily  produced  by  heating  a  bar  of  steel,  containing  0.50  per  cent 
carbon  or  more,  white  hot  at  one  end  and  quenching  it  in  waterj-when  at  some  dis- 
tance from  the  heated  end  the  temperature  must  necessarily  have  been  such  as  to 
produce  troostite.  This  can  generally  be  detected  by  means  of  a  file,  the  martensitic 
portion  of  the  bar  being  too  hard  to  be  marked  while  the  troostitic  part,  although 
hard,  can  be  scratched.  The  sorbitic  and  pearlitic  portions  are  decidedly  softer. 

Properties  of  Troostite.  —  It  will  be  obvious  from  the  foregoing  description  of  the 
nature  and  formation  of  troostite  that  its  physical  properties  must  be  intermediate 
between  those  of  martensite  and  of  sorbite.  For  like  carbon  content  troostite  is 
softer  and  more  ductile  than  martensite  but  harder  and  less  ductile  than  sorbite.  It 
will  be  shown  that  at  some  400  deg.  C.  it  begins  to  be  transformed  into  sorbite. 

Etching  of  Troostite.  —  Troostite  is  colored  decidedly  darker  than  any  other  con- 
st it  uent  by  the  ordinary  etching  reagents.  While  dilute  alcoholic  solutions  of  nitric, 
picric,  or  hydrochloric  acid  yield  satisfactory  results,  Kourbatoff's  reagent  is  pre- 
ferred by  some. 

Structure  of  Troostite.  —  Troostite  generally  occurs  as  dark-colored,  irregular 
areas,  representing  sections  through  nodules  generally  accompanied  by  martensite  or 
sorbite  or  both  or  as  membranes  surrounding  martensite  grains  (Figs.  282  to  287).  On 
deeper  etching  the  martensitic  structure  of  the  matrix  is  generally  revealed  as  shown 
in  Figure  286.  In  hypo-eutectoid  steel  free  ferrite,  and  in  hyper-eutectoid  steel 
free  cementite,  may  also  be  present  and,  indeed,  even  well-developed  pearlite  (Fig. 
287).  Osmond  describes  the  structure  of  troostite  as  "almost  amorphous,  slightly 
granular,  and  mammilated."  The  steel  shown  in  Figure  287  was  quenched  during 
its  critical  range  and  contains  martensite  (bright),  troostite  (dark),  sorbite  (lighter 
than  troostite  and  ill-defined),  and  pearlite  (laminated). 

Sorbite.  —  Sorbite  is  not,  properly  speaking,  a  constituent  of  hardened  steel.  It 
seems  appropriate,  however,  to  again  mention  it  here  seeing  that  it  constitutes  the 
connecting  link  between  annealed  (pearlitic)  steels  and  hardened  (troostito-marten- 
sitic)  steels,  and  also  because  it  results  from  the  transformation  of  troostite,  thus 
completing  the  various  stages  assumed  by  iron-carbon  alloys  in  passing  from  the 
condition  of  austenite,  stable  above  the  range,  to  that  of  pearlite,  stable  below  the 
range.  These  stages  are  (1)  austenite,  (2)  martensite,  (3)  troostite,  (4)  sorbite,  and 
(5)  pearlite. 

Sorbite  is  now  generally  regarded  as  an  uncoagulated  mixture  of  the  constituents 
of  troostite  and  of  pearlite;  it  apparently  contains  (1)  some  hardening  carbon,  i.e. 
carbon  or  FeaC  dissolved  in  beta  iron,  hence  the  greater  hardness  and  strength  of 
sorbite  compared  to  the  hardness  and  strength  of  pearlite,  (2)  a  considerable  quan- 
tity of  alpha  iron,  hence  its  magnetism  and  relative  softness,  and  (3)  a  considerable 
quantity  of  crystallized  FesC  (cement  carbon)  as  proven  by  analysis.  While  sorbite 
probably  contains  the  same  constituents  as  troostite  it  holds  considerably  less  unde- 
composed  solid  solution  and  considerably  more  alpha  iron,  hence  it  is  much  softer 


290  CHAPTER  XVI  — THE  HARDENING  OF  STEEL 

and  more  ductile  than  troostite.  In  other  words  the  transformation  which  eventually 
must  lead  to  the  formation  of  pearlite  is  more  advanced  in  sorbite  than  it  is  in  troost- 
ite. The  nomenclature  committee,  already  referred  to,  describes  sorbite  as  follows: 
"Aggregate  ...  In  the  transformation  of  austenite,  the  stage  following  troostite 
.  .  .  and  preceding  pearlite.  Most  writers  believe  it  essentially  an  uncoagulated  con- 
glomerate of  irresoluble  pearlite  with  ferrite  in  hypo-  and  cementite  in  hyper-eutec- 
toid  steels  respectively." 

The  occurrence,  etching,  structure,  and  properties  of  sorbite  have  been  described 
in  Chapters  XIV  and  XV  when  it  was  shown  that  it  is  formed  (1)  in  small  pieces  of 
steel  cooling  in  the  air  from  above  their  critical  range,  (2)  in  larger  pieces  quenched 
in  oil  from  above  the  range,  or  (3)  in  small  pieces  quenched  in  water  from  near  the 


Fig.  287. —  Steel.  Carbon  0.80  per  cent. 
Quenched  in  the  critical  range.  Magnified 
430  diameters.  (Boylston.) 


bottom  of  the  range.  In  other  words  to  form  sorbite  we  must  so  regulate  the  cool- 
ing through  the  critical  range  that  it  is  allowed  to  form  but  prevented  from  further 
transformation  (into  pearlite).  It  will  be  seen  in  the  next  chapter  that  sorbite  is  also 
formed  on  tempering  austenitic,  martensitic,  and  troostitic  steels. 

By  its  physical  properties  sorbite  occupies  an  intermediate  position  between 
troostite  and  pearlite;  as  previously  mentioned  it  is  stronger,  harder,  and  less  ductile 
than  pearlite  but  softer  and  more  ductile  than  troostite. 

Troosto-Sorbite.  —  Kourbatoff  gives  the  name  of  "troosto-sorbite"  to  a  constituent 
associated  with  martensite  and  austenite  in  quenching,  from  a  high  temperature, 
steels  very  high  in  carbon.  It  is  not  clear  that  this  constituent  is  more  than  a  mixture 
of  troostite  and  sorbite.  We  may  talk  of  troosto-sorbite  as  we  do  of  a  greenish  blue 
tint  to  indicate  shades  intermediate  between  green  and  blue,  and  similarly  the  ex- 
pressions martenso-austenite,  troosto-martensite,  and  sorbitic-pearlite,  or  like  expres- 
sions, are  useful  and  their  meanings  obvious.  In  the  report  of  the  Committee  on  the 


CHAPTER  XVI  — THE   HARDENING   OF  STEEL  291 

Nomenclature  of  the  Microscopical  Constituents  of  Iron  and  Steel  of  the  International 
Association  for  Testing  Materials,  troosto-sorbite  is  thus  denned:  "Indefinite  aggre- 
gate, the  troostite  and  the  sorbite  which  lie  near  the  boundary  which  separates  these 
two  aggregates." 

Hardenite. — The  name  of  "hardenite"  is  frequently  given  both  to  austenite  and  to 
martensite  of  eutectoid  composition,1  i.e.  to  the  original  austenite  of  eutectoid  steel 
and  to  the  residual  austenite  of  hypo-  and  hyper-eutectoid  steel  after  rejection  of  the 
full  amount  of  free  ferrite  or  of  free  cementite.  In  other  words  Ihe^  name  is  applied 

(1)  to  the  condition  of  austenite  in  slowly  cooled  steels  immediately  preceding  its 
conversion  into  martensite  and  (2)  to  the  resulting  martensite  (necessarily  of  eutec- 
toid composition  if  the  cooling  to  the  range  has  been  sufficiently  slow).    It  is  unfor- 
tunate that  the  same  term  is  used  to  designate  both  austenite  and  martensite,  two 
apparently  sharply  different  constituents,  as  it  is  likely  to  lead  to  confusion.    Its  use 
should  be  restricted  to  the  designation  of  austenite  of  eutectoid  composition.    Giving 
it  this  meaning  it  will  be  apparent,  as  later  explained,  that  hardenite  possesses  maxi- 
mum hardening  power  and,  therefore,  that  steel  made  up  exclusively  of  hardenite, 
i.e.  eutectoid  steel,  possesses  maximum  hardening  power. 

Rate  of  Cooling  through  Critical  Range  vs.  Structure  of  Steel.  —  It  has  been  made 
clear  in  the  foregoing  pages  (1)  that  in  order  to  retain  some  austenite  in  the  cold  the 
metal  should  be  highly  carburized  and  very  quickly  cooled  from  a  high  temperature, 

(2)  that  pearlite  is  produced  by  very  slow  cooling  through  the  critical  range,  and 

(3)  that  in  order  to  cause  the  formation  of  any  of  the  three  recognized  transition  con- 
stituents, namely  martensite,  troostite,  and  sorbite,  the  steel  should  be  cooled  through 
its  critical  range  in  such  a  way  as  to  allow  the  formation  of  the  desired  constituent 
while  preventing  its  further  transformation  as,  for  instance,  (a)  by  cooling  the  metal 
slowly  to  that  portion  of  the  range  in  which  the  constituent  is  formed  and  then  quickly 
to  atmospheric  temperature  or  (fc)  by  cooling  the  metal  through  its  range  at  a  uni- 
form speed  but  so  regulated  that  the  transformation  of  austenite  proceeds  only  to 
the  desired  extent,  to  wit,  cooling  in  water  for  martensite,  in  oil  for  sorbite. 

An  attempt  has  been  made  in  Figure  288  to  give  a  graphical  illustration  of  the 
cooling  conditions  needed  for  the  production  of  the  various  constituents  of  steel. 
Its  interpretation  will  be  obvious.  The  critical  range,  or  rather  the  lower  critical 
point,  Ari  or  Ar3.2.i,  is  represented  as  covering  a  considerable  range  of  temperature 
so  as  to  afford  the  necessary  room  for  the  diagrammatical  representation  of  the  for- 
mation, within  that  range,  of  the  transition  constituents.  The  diagram  indicates 
that  as  the  metal  cools  slowly  through  its  range  it  does  not  pass  abruptly  from  an 
austenitic  to  a  martensitic  condition  and  then  to  troostite,  etc.,  but  that  these  trans- 
formations are,  on  the  contrary,  gradual,  the  following  types  of  structure  being  formed, 
theoretically  at  least:  austenite,  austenite  plus  martensite,  martensite,  martensite 
plus  troostite,  troostite,  troostite  plus  sorbite,  sorbite,  sorbite  plus  pearlite,  and 
pearlite.  The  transformations  depicted  refer  to  eutectoid  steel  or  to  the  residual  aus- 
tenite (necessarily  of  eutectoid  composition)  of  hypo-  and  hyper-eutectoid  steel  formed 
on  slow  cooling  to  Ari  after  rejection  of  free  ferrite  or  free  cementite.  In  the  case  of 
these  steels,  therefore,  free  ferrite  or  free  cementite  is  present  in  the  above  structures 

1  Originally  the  name  hardenite  was  applied  by  Howe  to  austenite  and  martensite  of  any  com- 
position (1888).  Osmond  used  it  to  designate  austenite  saturated  with  carbon  (1897).  Both  these 
meanings  have  been  withdrawn  by  their  proposers.  Arnold  calls  hardenite  the  carbide  Fe2<C  which 
he  believes  exists  above  the  critical  range. 


292 


CHAPTER   XVI  — THE   HARDENING   OF   STEEL 


unless,  indeed,  cooling  between  Ar3  and  Arj,  or  between  Arcm  and  Ar!  has  been  so  rapid 
as  to  prevent  their  separation.  While,  theoretically,  very  quick  cooling  from  a  to  at- 
mospheric temperature  should  retain  the  steel  in  its  austenitic  condition,  even  under 
the  most  favorable  conditions,  this  can  be  done  but  partially,  considerable  marten- 
site  being  produced.  Cooling  slowly  to  m  and  then  quickly  should  produce  marten- 
site,  while  slow  cooling  to  t  or  .s  followed  by  quick  cooling  should  produce,  respectively, 
troostite  and  sorbite.  Slow  cooling  to  /),  followed  or  not  by  quick  cooling,  results  in 
the  formation  of  pearlite.  Slow  cooling  to  intermediate  points  between  m  and  t  or 
t  and  s,  etc.,  should,  theoretically,  cause  the  formation  of  martensite  and  troostite. 
troostite  and  sorbite,  etc. 


& 


**, 


/  /  /  / 


P 


/  / 

T   M  A 


A — Austenite 
M — Martensite 
T— Troostite 
S — Snrbite 
P — Pearlite 


Ausfenife 


A   M  T 


Fig.  288.  —  Diagram  depicting  the  formation  of  austenitc,  martensite,  troostite,  sorbite,  anil  pearlite 

in  steel  cooling  through  its  critical  range. 


These  conditions  may  be  realized  in  the  same  piece  of  steel  by  heating  one  end  of 
a  steel  bar,  preferably  of  eutectoid  or  hyper-cut  ectoid  composition,  well  above  the 
critical  range  and  quenching  the  whole  bar  in  ice-cold  water.  It  is  evident  that,  since 
at  the  time  of  quenching  the  temperature  of  the  bar  decreased  gradually  from  the 
hot  to  the  cold  end,  a  portion  of  the  bar  must  have  been  quenched  while  in  the  mar- 
tensitic  condition,  another  while  in  the  troostitic  condition,  etc.  The  preparation 
and  microscopical  examination  of  longitudinal  sections  through  the  center  of  the  bar 
should  reveal  the  existence  of  the  various  constituents  indicating  as  many  stages  in 
the  transformation  of  austenite. 

On  the  left  of  the  diagram  five  lines  starting  from  the  point  R  above  the  critical 


CHAPTER   XVI  — THE   HARDENING   OF   STEEL  293 

range  represent  coolings  through  the  range  at  different  speeds  as,  for  instance,  (1)  very 
quickly  in  ice-cold  water,  (2)  quickly  in  water,  (3)  less  quickly  in  oil,  (4)  slowly  in  air, 
and  (5)  very  slowly  in  furnace,  resulting,  respectively,  in  the  formation  of  austenite 
(or  rather  austenite  and  martensite),  martensite,  troostite,  sorbite,  and  pearlite  or, 
more  frequently,  of  mixtures  of  two  or  even  three  of  these  constituents. 

These  conditions  may  be  realized  in  the  same  piece  of  metal  by  heating  a  steel 
bar  of  considerable  cross  section  (not  less  than  one  inch  in  diameter)  and  preferably 
of  eutectoid  or  hyper-eutectoid  composition  to  a  temperature  well  above  the  critical 
range  and  quenching  it  in  water.  The  cooling  should  be  rapid  enough  to  cause  the 
formation  of  martensite  near  the  outside  of  the  piece  while  the  cooling  of  the  center 
should  be  so  slow  (because  of  the  size  of  the  bar)  as  to  permit  the  formation  of  pear- 
lite.  Between  the  martensitic  shell  and  the  pearlitic  core  the  metal  should  be  com- 
posed of  the  other  transition  constituents,  that  is  starting  from  the  center,  of  sorbite 
and  then  of  troostite.  The  microscopical  examination  of  a  cross  section  through  the 
bar  should  reveal  this  gradual  change  of  structure  from  center  to  outside. 

The  formation  of  a  transition  constituent  through  the  tempering  of  a  constituent 
representing  a  less  advanced,  and  therefore  less  stable,  stage  of  transformation  has 
already  been  alluded  to  and  will  be  considered  at  greater  length  in  the  next 
chapter. 

Are  the  Transition  Stages  Distinct  Constituents?  —  It  would  appear  from  our 
consideration  of  the  formation  of  the  transition  constituents  that  they  must  represent 
as  many  stages  in  the  progressive  transformation  of  austenite  and  that  sharp  lines  of 
demarcation  between  them  are  not  to  be  expected  or,  rather,  that  they  must  be  linked 
together  by  an  unbroken  chain  of  transition  stages  just  as  the  blue  and  yellow  colors 
are  connected  through  an  unbroken  series  of  bluish,  greenish,  and  yellowish  shades. 
This  logical  inference,  however,  is  not  supported  by  microscopical  evidences  in  the 
cases  of  austenite,  martensite,  and  troostite  for  these  constituents  are  sharply  differ- 
entiated from  each  other  whenever  they  occur  together.  Stages  or  structural  condi- 
tions representing  the  gradual  transformation  of  austenite  into  martensite  or  of  mar- 
tensite into  troostite  are  not  observed;  it  is  as  if  these  transformations  had  actually 
taken  place  by  rather  sudden  steps. 

The  transformations  of  troostite  into  sorbite  and  of  sorbite  into  pearlite,  on  the 
contrary,  appear  to  be  very  gradual  and  easy  to  follow.  In  other  words  while  troost- 
ite, sorbite,  and  pearlite  are  readily  distinguishable,  each  having  marked  charac- 
teristic of  its  own,  structural  arrangements  are  frequently  observed  which  undoubt- 
edly correspond  1o  intermediate  stages. 

The  five  constituents  of  steel,  resulting  from  various  modes  and  rates  of  cooling, 
might  be  compared  to  a  spectrum  of  five  elementary  colors,  i.e.  violet,  blue,  yellow 
orange,  and  red,  with  gaps  existing  between  the  first  three  (our  austenite,  martensite, 
and  troostite)  while  the  third  and  the  fifth,  yellow  and  red  (troostite  and  pearlite) 
are  closely  linked  together  by  the  fourth,  orange  (our  sorbite),  and  an  infinity  of  in- 
termediate shades. 

Metarals  and  Aggregates.  —  Howe  suggests  dividing  all  microscopical  constit- 
uents of  iron  and  steel  into  (1)  metarals  and  (2)  aggregates. 

These  he  describes  as  follows:  "Metarals,  true  phases  like  the  minerals  of  nature. 
These  are  like  definite  chemical  compounds,  or  solid  solutions,  and  hence  consisting 
of  definite  substances  in  varying  proportions  .  .  .  Aggregates,  like  the  petrographic 
entities  as  distinguished  from  the  true  minerals.  These  mixtures  may  be  in  definite 


294  CHAPTER  XVI  — THE  HARDENING  OF  STEEL 

proportions,  i.e.  eutectic  or  eutectoid  mixtures  (ledeburite,  pearlite,  steadite)  or  in 
indefinite  proportions  (troostite,  sorbite)."1  Under  these  two  headings  the  constit- 
uents of  iron-carbon  alloys  would  be  classified  as  follows: 

Metarals:  ferrite,  cementite,  austenite,  graphite. 

Aggregates:  pearlite,  sorbite,  troostite,  ledeburite,  steadite. 

Opinions  differ  as  to  the  nature  of  martensite;  if  it  is  a  solid  solution  it  is  a 
metaral,  if  not  it  must  be  classified  with  the  aggregates.  Should  we  recognize  the 
existence  of  solid  colloidal  solutions,  it  is  not  clear  whether  these  should  be  grouped 
with  the  metarals  or  should  form  a  distinct  class  between  the  metarals  and  the  aggre- 
gates. 

Hardening  Eutectoid  Steel.  —  It  will  now  be  profitable  to  consider  separately 
the  hardening  of  eutectoid,  hyper-eutectoid,  and  hypo-eutectoid  steel.  In  hardening 
eutectoid  steel  the  metal  should  be  heated  through  its  critical  range,  i.e.  through  its 
single  critical  point  Ac3.2.i.  By  so  doing  we  confer  upon  it  full  hardening  power  and 
finest  possible  structure.  The  steel  should  then  be  cooled  from  that  temperature  as 
promptly  as  possible  avoiding  heating  much  above  the  range  or  long  exposure  at  any 
temperature  above  the  range,  as  either  procedure  would  tend  to  increase  the  grain 
size  of  the  metal.  A  temperature  of  some  775  to  825  deg.  C.  will  generally  be,  there- 
fore, the  best  temperature  to  which  to  heat  and  from  which  to  cool  eutectoid  steel 
for  the  purpose  of  hardening.  By  this  treatment  the  steel  passes  from  a  finely  aus- 
tenitic  to  a  finely  martensitic  or  troostito-martensitic  condition. 

Hardening  Hyper-Eutectoid  Steel.  —  Let  us  assume  a  steel  containing  1.25  per 
cent  carbon  and,  therefore,  composed  approximately  of  93  per  cent  of  pearlite  and 
7  per  cent  of  free  cementite.  Upon  heating  this  steel  through  its  lower  point  Ac3.2.i 
its  93  per  cent  of  pearlite  are  converted  into  93  per  cent  of  austenite  possessing  hard- 
ening power,  but  the  metal  still  contains  7  per  cent  of  free  cementite  deprived  of 
hardening  power.  If  we  heat  it  past  its  upper  point  Accm  the  free  cementite  is  ab- 
sorbed and  the  whole  mass  becomes  austenitic.  A  little  reflection  will  show,  however, 
that  the  steel  should  be  quenched  as  soon  as  it  rises  above  the  point  Acs.o.i,  because  we 
then  produce,  theoretically  at  least,  93  per  cent  of  fine  grained  martensite  while  re- 
taining, to  be  sure,  the  original  7  per  cent  of  cementite,  but  as  this  constituent  is 
harder  than  martensite  its  presence  adds  to,  rather  than  takes  away  from,  the  hard- 
ness of  the  quenched  metal.  Should  we,  on  the  contrary,  heat  to  above  Accm  before 
quenching  the  whole  mass  would  be  converted  into  martensite  but  it  would  be  less 
hard  if  anything  than  the  metal  quenched  at  a  lower  temperature  while  its  structure 
would  be  coarser  and  the  danger  of  cracking  the  objects  in  the  quenching  bath  would 
be  greater.  The  best  hardening  temperature  for  hyper-eutectoid  steels,  therefore, 
is  the  same  as  that  for  eutectoid  steel,  namely,  some  775  to  825  deg.  C. 

The  following  treatment  has  been  described  for  hardening  hyper-eutectoid  steel : 
Heat  above  Acm  to  cause  the  absorption  of  the  free  cementite  and  cool  in  boiling  water 
or  lead,  thereby  preventing  the  excess  cementite  from  re-forming  although  the  quench- 
ing is  too  mild  to  produce  cracks  or  serious  distortion;  the  piece  should  then  be  re- 
heated slightly  above  Aci  and  quenched  in  the  usual  way. 

1  Howe  further  writes:  "Many  true  minerals,  such  as  mica,  felspar,  and  hornblende,  are  divisible 
into  several  different  species.  Such  minerals  are  definite  chemical  compounds,  in  which  one  element 
may  replace  another.  Others,  such  as  obsidian,  are  solid  solutions  in  varying  proportions  and  in  these 
also  one  element  may  replace  another.  Metarals  like  minerals  differ  from  aggregates  in  being  sever- 
ally chemically  homogeneous." 


CHAPTER  XVI  — THE   HARDENING   OF   STEEL  295 

The  structure  of  a  properly  hardened  hyper-eutectoid  steel  is  shown  in  Figure 
289.  Like  hardened  eutectoid  steel  it  consists  of  very  fine  martensite. 

Hardening  Hypo-Eutectoid  Steel.  —  Let  us  take  a  steel  containing  some  0.50  per 
cent  carbon  and  exhibiting  therefore  the  points  AI  and  A3.2.  Such  steel  is  made  up 
of  60  per  cent  of  pearlite  and  40  per  cent  free  ferrite.  Upon  heating  it  through  its 
Aci  point  the  pearlite  is  converted  into  austenite,  so  that  at  this  temperature  some 
60  per  cent  of  the  mass  of  the  metal  is  endowed  with  hardening  power.  Should  we 
quench  this  steel,  therefore,  as  soon  as  it  rises  above  its  lower  critical  point  but  60 
per  cent  of  its  bulk  would  be  hardened;  it  would  still  retain  40  per  cent  of  soft  ferrite. 
If,  on  the  contrary,  the  heating  be  carried  to  just  above  Acs.2  the  free  ferrite  is  ab- 
sorbed by  the  austenite  and  the  whole  mass  becomes  hardenable.  Upon  quenching 


Fig.  289.  —  Steel.    Carbon  1.10  per  cent.    Magnified  100  diameters.    Quenched 
in  water  from  above  its  critical  range.     (Boylston.) 


the  steel  from  that  temperature  its  entire  bulk  may  be  converted  into  martensite  or 
troostito-martensite,  according  to  rate  of  cooling.  While  this  martensite  will  not  be 
quite  as  hard  as  the  martensite  produced  by  quenching  from  just  above  Aci  the  metal 
as  a  whole  will  be  harder  and  of  a  more  uniform  and  finer  structure  because  of  the  ab- 
sence of  free  ferrite,  or  at  least  of  any  considerable  amount  of  it.  It  follows  from  these 
considerations  that  for  the  purpose  of  hardening,  hypo-eutectoid  steel  should  be 
quenched  from  just  above  its  upper  critical  point,  namely,  Ac3.2  or  Ac3  (825  to  925 
deg.  C.  according  to  carbon  content).  Hypo-eutectoid  steel  containing  very  little 
carbon,  say  less  than  0.25  per  cent,  cannot  be  very  materially  hardened  by  the  or- 
dinary quenching  methods  because  of  the  large  amount  of  soft  ferrite  which  it  con- 
tains in  excess  of  the  eutectoid  ratio  and  wh'ieh  cannot  be  retained  in  solution,  even 
on  very  quick  cooling  (see  Chapter  XV,  Fig.  229).  The  structure  of  steel  containing 
about  0.25  per  cent  carbon  and  quenched  in  water  is  shown  in  Figure  290. 


296  CHAPTER  XVI  — THE  HARDENING  OF  STEEL 

Steel  of  Maximum  Hardening  Power.  —  From  the  above  considerations  it  will  be 
obvious  that  the  hardening  of  steel  consists  in  preventing  the  formation  of  relatively 
soft  pearlite  and  in  causing,  instead,  the  formation  and  retention  of  hard  martensite 
or  troostite  or,  more  often,  of  both.  It  follows  from  this  that  the  steel  possessing 
maximum  hardening  power  must  be  that  steel  which  in  slow  cooling  would  contain 
most  pearlite,  namely,  eutectoid  steel.  It  does  not,  of  course,  mean  that  quenched 
eutectoid  steel  is  harder  than  quenched  hyper-eutectoid  steel  but  merely  that  the 
increased  hardness  produced  by  quenching  is  greatest  in  the  case  of  eutectoid  steel. 
Quenched  "hyper-eutectoid  steel  is  harder  than  quenched  eutectoid  steel  because  of 
the  presence  in  the  former  of  some  free  cementite  or  of  more  highly  carburized  mar- 


Fig.  290.  —  Steel.  About  0.25  per  cent  carbon. 
Heated  to  1000  cleg.  C.  and  quenched  in  water. 
Magnified  100  diameters.  (W.  B.  Byers  in  the 
author's  laboratory.) 

tensite  but  the  difference  of  hardness  between  the  two  steels  is  greater  before 
quenching. 

Hardening  Large  Pieces.  —  In  hardening  pieces  of  considerable  cross  section  it  is 
evident  that  the  central  portions  will  not  cool  as  quickly  as  the  outside  and  will  not, 
therefore,  be  as  hard.  Indeed  the  center  may  cool  so  slowly  that  it  will  fail  to  harden 
at  all.  The  limitation  of  the  hardening  process,  as  applied  to  large  pieces,  will,  there- 
fore, be  evident.  It  is  seldom  desirable,  however,  to  harden  large  pieces  to  their 
'very  core.  When  large  steel  objects  are  to  be  hardened,  as  for  instance  in  the  case  of 
armor  plates,  superficial  hardness  only  is  desired,  or  at  least  hardness  penetrating  to 
but  a  relatively  small  depth,  and  this  is  readily  secured  through  the  case  hardening 
process. 

The  influence  of  mass  in  cooling  for  hardening  is  well  illustrated  by  Le  Ohatelier 
who  states  that  a  steel  bar  containing  1.00  per  cent  carbon  and  measuring  10  mm.  in 
diameter  when  quenched  in  water  is  wholly  martensitic  whereas  the  same  steel  in 
the  form  of  bars  50  mm.  in  diameter  in  entirely  troostitic  after  quenching,  interme- 
diate sizes  being  mixtures  of  martensite  and  troostite. 


CHAPTER  XVI  — THE   HARDENING   OF  STEEL  297 

Hardening  and  Tempering  in  One  Operation.  —  A  peculiar  but  frequent  way  of 
hardening  tools  consists  in  heating  the  tool  to  the  proper  temperature  and  then  cool- 
ing quickly  to  a  black  heat  only  that  portion  of  it  which  should  be  hard,  while  keep- 
ing the  other  portion  out  of  the  bath  and,  therefore,  at  a  high  temperature.  Upon 
withdrawing  the  tool  from  the  bath  the  heat  stored  away  in  the  hot  portion  diffuses 
to  the  cooled  portion  which  is  in  this  way  reheated,  that  is,  tempered,  as  later  ex- 
plained, the  amount  of  tempering  being  regulated  at  will  by  again  quenching  the 
metal  as  soon  as  the  desired  tempering  color  has  been  obtained —  _ 


CHAPTER  XVII 

THE  TEMPERING   OF  HARDENED  STEEL 

Steel  that  has  been  hardened  by  rapid  cooling  from  above  its  critical  range,  as 
explained  in  the  preceding  chapter,  is  often  harder  than  necessary  and  generally  too 
brittle  for  most  purposes  besides  being  under  severe  internal  strain.  In  order  to  re- 
lieve the  strains  and  decrease  its  brittleness,  that  is,  to  toughen  it  without  very 
material  diminution  of  hardness,  the  metal  is  generally  "tempered,"  that  is,  re- 
heated to  a  temperature  considerably  below  its  critical  range.  This  operation  is 
called  tempering  because  it  somewhat  mitigates  or  tempers  the  effects  of  the  previous 
hardening  treatment.  Mathews  calls  attention  to  the  importance  of  tempering  im- 
mediately after  hardening,  lest  the  severe  strains  existing  in  the  steel  result  in  the 
destruction  by  breaking  of  expensive  tools. 

Tempering  Temperatures.  —  The  hardening  of  steel  causes  increased  hardness, 
brittleness,  and  elastic  limit,  all  of  which  are  somewhat  lowered  by  the  tempering 
operation.  The  effect  of  tempering  begins  to  be  noticeable  at  about  100  cleg.  C. 
and  increases  in  intensity  as  the  temperature  rises,  until  finally  at  some  600  deg.  the 
metal  assumes  again  the  physical  properties  characteristic  of  the  unhardened  con- 
dition. The  temperature  to  which  hardened  steel  should  be  heated  for  tempering 
varies,  therefore,  with  the  use  to  which  it  is  destined.  If  it  is  desired  to  retain  the 
greatest  possible  hardness,  necessarily  with  its  accompanying  brittleness,  the  steel 
should  be  reheated  but  slightly  above  200  deg.  C.  as,  for  instance,  in  tempering  razor 
blades  when  extreme  hardness  is  essential  and  brittleness  relatively  immaterial.  If, 
on  the  contrary,  considerable  toughness  is  indispensable,  at  the  necessary  sacrifice  of 
some  hardness  the  steel  should  be  tempered  to  some  300  deg.  C.  or  even  to  a  higher 
temperature.  The  great  majority  of  tools  are  tempered  between  200  and  300  deg. 

Tempering  Colors.  —  Hardened  steel  objects  subjected  to  tempering  being  gen- 
erally quite  bright  and  their  heating  being  generally  conducted  in  an  oxidizing  atmos- 
phere, very  thin  films  of  oxides  form  upon  their  surfaces.  The  colors  of  these  films 
vary  with  the  temperature,  that  is,  to  each  tempering  temperature  corresponds  a  cer- 
tain color,  and  blacksmiths  generally  depend  upon  these  colors  for  the  tempering  of 
their  tools,  the  use  of  pyrometers  for  this  operation  being  far  from  general.  Accord- 
ing to  Howe  the  tempering  colors  and  corresponding  temperatures  are  as  follows: 

Pale  yellow 220  deg.  C.  or  428  deg.  F. 

Straw      , 230  """  446  "      " 

Golden  yellow 243  """  469  "      " 

Brown 255  """  491  "      " 

Brown  dappled  with  purple    ....  265  "     "    "  509  "     " 

Purple 277  """  531  "      " 

Bright  blue 288  """  550  "" 

Pale  blue 297  """  567  "     " 

Dark  blue 316  """  600  "      " 

In  order  that  the  tempering  colors  may  be  plainly  seen  the  steel  objects  should  be 
smooth  and  bright,  preferably  polished. 

298 


CHAPTER  XVII  — THE   TEMPERING   OF   HARDENED   STEEL  299 

Time  at  Tempering  Temperature.  —  It  is  the  common  belief  that  once  the  de- 
sired temperature  is  obtained,  as  indicated  by  the  color,  little  is  to  be  gained  by  main- 
taining the  steel  at  that  temperature  any  length  of  time  on  the  ground  that  it  will  not 
result  in  producing  additional  tempering.  The  tempering  of  steel  has  been  compared 
to  the  releasing  of  a  spring  permitting  a  certain  structural  rearrangement,  that  is,  a 
certain  tempering  of  the  metal  at  any  temperature  but  not  more.  To  produce  addi- 
tional tempering  the  temperature  must  be  increased,  that  is,  the  spring  must  be  fur- 
ther released.  Recent  investigations,  however,  have  shown  that  the  maintenance  of 
hardened  steel  at  a  certain  tempering  temperature  often  does  produce  additional 
tempering  effect.  It  was  further  ascertained  that  the  color,  instead  of  remaining  un- 
changed at  any  given  temperature,  advances  in  the  tempering  color  scale  as  it  would 
with  increasing  temperature.  In  other  words,  the  tempering  colors,  contrary  to  the 
view  generally  held,  are  not  an  absolute  criterion  by  which  to  judge  of  the  tempera- 
ture of  the  steel,  since  they  vary  with  the  length  of  time  during  which  the  steel  is  kept 
at  any  temperature.  These  experiments  seem  to  show,  however,  that  the  amount  of 
tempering  effected  is  closely  related  to  the  color,  that  is,  that  to  each  shade  corre- 
sponds a  certain  amount  of  tempering.  It  should,  however,  be  borne  in  mind  that 
these  colors,  with  the  corresponding  tempering  they  imply,  may  be  obtained  in  two 
ways,  (1)  through  short  exposure  at  a  certain  temperature  and  (2)  through  longer  ex- 
posure at  lower  temperatures.  The  same  amount  of  tempering,  for  instance,  would 
result  (a)  from  heating  hardened  steel  to  288  deg.,  when  its  color  is  bright  blue,  fol- 
lowed by  immediate  cooling  and  (6)  from  heating  it  to  255  deg.,  when  its  color  is 
brown,  and  maintaining  it  at  that  temperature  until  its  color  becomes  bright  blue. 
Unless  baths  kept  at  constant  temperatures  are  used,  however,  it  is  evident  that  in 
practise  the  steel  should  be  quenched  as  soon  as  the  desired  color  is  produced  and 
while  its  temperature  is  rising,  because  it  is  simpler  and  more  convenient  to  produce 
that  color  on  a  rising  temperature  than  by  maintaining  the  metal  at  a  constant 
temperature. 

Some  writers  doubt  the  existence  of  so  close  a  relation  between  the  color  and  the 
resulting  tempering.  According  to  Barus  and  Strouhal  to  each  tempering  tempera- 
ture corresponds  a  maximum  tempering  effect,  which  is  the  more  quickly  reached  the 
higher  the  temperature.  At  100  deg.,  for  instance,  the  maximum  effect  was  not  ob- 
tained after  one  hour  although  maintaining  the  steel  at  that  temperature  two  more 
hours  had  but  little  additional  effect.  At  200  deg.  the  maximum  effect  was  obtained 
in  ten  minutes,  while  at  300  deg.  one  minute  was  sufficient. 

Mathews  tempered  3  hardened  pieces  of  the  same  steel  in  a  salt  bath  at  422  deg. 
C.  keeping  them  at  that  temperature  respectively  for  8,  20,  and  40  minutes  and  ob- 
tained Brinell  hardness  numbers  of  425,  390,  and  340  respectively. 

Rate  of  Cooling  from  Tempering  Temperature.  —  Once  the  desired  amount  of 
tempering  is  effected,  as  indicated  by  the  color  or  otherwise,  the  rate  of  cooling  to 
atmospheric  temperature  appears  to  be  quite  immaterial.  In  practise  the  piece  is 
generally  quenched,  merely  for  convenience.  The  theory  is  that  while  by  keeping  the 
metal  at  a  certain  temperature  its  tempering  may  be  carried  farther,  on  cooling  tem- 
pering ceases,  for  the  spring  is  now  tightened,  to  use  the  simile  already  referred  to, 
so  that  the  rate  of  cooling  is  without  influence. 

Hardening  and  Tempering  Combined.  — •  A  method  of  hardening  and  tempering 
combined,  frequently  employed  when  but  one  end  of  a  tool  must  be  hardened,  as  in 
the  case  of  chisels  and  drills,  has  been  described  in  the  preceding  chapter.  It  consists 


300  CHAPTER  XVII  — THE   TEMPERING   OF   HARDENED   STEEL 

in  heating  the  tool  above  its  critical  range,  quenching  that  portion  only  which  is  to 
be  hard,  removing  it  from  the  bath,  and  allowing  the  heat  stored  in  the  unquenched 
portion  to  heat  by  conduction  the  quenched  part  until  the  desired  temper  color  is 
obtained,  when  it  is  again  quenched  lest  the  tempering  be  carried  too  far. 

Explanation  of  the  Tempering  of  Steel.  —  The  theories  accounting  for  the  tem- 
pering of  steel  will  be  considered  in  the  next  chapter  together  with  the  hardening 
theories.  It  will  suffice  for  the  present  to  point  out  that  hardened  steel  is  generally 
considered  to  be  in  an  unstable  condition  and,  therefore,  eager  to  return  to  a  more 
stable  form  and  actually  undergoing  this  change  whenever  given  an  opportunity, 
that  is  on  raising  its  temperature.  At  atmospheric  temperature  the  passage  of  an 
unstable  martensitic  condition  into  a  more  stable  troostitic  or  sorbitic  form  is 
prevented  by  the  rigidity  of  the  metal.  A  slight  heating,  however,  produces  some 
plasticity  and  such  transformation  takes  place  to  a  small  extent.  On  increasing  the 
temperature  the  rigidity  diminishes  farther  and  the  transformation  advances.  The 
tempering  of  hardened  steel,  in  other  words,  is  due  to  its  transformation  from  an  un- 
stable condition  to  one  more  stable.  It  will  be  seen  that  at  some  600  deg.  C.  the 
metal  assumes  a  stable  condition,  i.e.  is  fully  tempered. 

Tempering  Austenitic  Steels.  —  It  has  been  explained  in  the  preceding  chapter 
that  austenite,  martensite,  troostite,  and  even  sorbite  were  the  constituents  formed 
in  hardened  steel  according  to  the  rapidity  with  which  the  metal  is  cooled  through 
and  below  its  critical  range.  It  will  now  be  instructive  to  consider  separately  the 
tempering  of  steels  having  these  different  types  of  structure. 

Austenitic  carbon  steel,  as  already  stated,  is  not  a  commercial  article,  as  it  re- 
quires for  its  production,  at  least  in  the  absence  of  a  large  proportion  of  manganese, 
the  presence  of  much  carbon,  an  excessively  high  quenching  temperature,  and  a 
quenching  bath  at  a  very  low  temperature.  And  even  when  these  conditions  pre- 
vail, only  one  half  or  so  of  the  bulk  of  the  steel  can  be  retained  in  an  austenitic  con- 
dition, the  other  half  being  martensitic.  The  tempering  of  austenite  should,  never- 
theless, be  considered.  Since  in  austenitic  steel  the  condition  of  the  metal  stable  only 
above  the  critical  range  has  been  retained  in  the  cold,  it  follows  that  cold  austenitic 
steel  must  be  in  a  very  unstable  condition.  At  atmospheric  temperature  the  rigidity 
of  the  metal  is  so  great  that  a  return  to  a  more  stable  form  is  not  possible  but,  on  heat- 
ing it  very  slightly,  sufficient  plasticity  is  produced  to  permit  a  partial  transformation 
of  austenite.  This  partial  transformation,  theoretically  at  least,  should  result  in  the 
formation  of  martensite,  troostite,  and  sorbite  in  the  order  named  as  the  tempering 
temperature  increases.  This  has  been  depicted  in  I,  Figure  291.  In  this  diagram  it  is 
shown  (1)  that  as  soon  as  austenite  is  heated  above  atmospheric  temperature  it  begins 
to  be  converted  into  martensite,  (2)  that  it  is  entirely  converted  into  martensite  at 
200  deg.  C.,  (3)  that  in  heating  above  200  deg.  martensite  begins  to  pass  to  troostite, 
(4)  that  at  400  deg.  the  transformation  of  martensite  into  troostite  is  complete,  (5)  that 
above  400  deg.  troostite  is  gradually  converted  into  sorbite,  (6)  that  at  600  deg.  C. 
the  transformation  of  troostite  into  sorbite  is  complete,  and  (7)  that  the  sorbite  con- 
dition is  the  final  condition  acquired  by  hardened  steel  when  reheated  close  to,  but 
below,  its  critical  range.  While  it  is  certain  that  lamellar,  i.e.  true  pearlite,  cannot  be 
formed  by  reheating  hardened  steel,  it  has  been  shown  that  long  heating  of  sorbite 
near  the  critical  range,  that  is,  between  600  and  700  deg.  ('.,  will  result  in  the  forma- 
tion of  granular  pearlite  brought  about  as  explained  in  another  chapter  by  the  spheroi- 
dizing  of  the  cementite.  The  tempering  of  austenite  depicted  in  I,  Figure  291,  rep- 


CHAPTER   XVII  — THE   TEMPERING    OF   HARDENED   STEEL 


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302  CHAPTER  XVII  — THE   TEMPERING   OF   HARDENED   STEEL 

resents  the  transformation  which,  according  to  theoretical  consideration,  should  be 
expected  to  take  place.  Most  observers,  however,  report  that  on  tempering  austenite 
it  is  at  once  converted  into  troostite  as  soon  as  the  rigidity  of  the  steel  has  been  suffi- 
ciently relaxed,  the  martensitic  stage  not  being  assumed.  This  is  shown  diagram- 
matically  in  II,  Figure  291.  Here,  again,  it  is  indicated  that  at  400  deg.  the  steel  is 
entirely  troostitic  and  at  600  deg.  entirely  sorbitic.  Benedicks  thinks  that  it  is  quite 
possible  that  austenite  is  first  transformed  into  martensite  but  that  the  resulting 
martensite  is  so  readily  and  quickly  converted  into  troostite  that  its  short  existence 
easily  escapes  observation.  His  belief  is  based  upon  the  following  considerations: 
(1)  the  austenite  retained  in  high  carbon  steel  by  very  sudden  cooling  is  subjected  to 
great  pressure  caused  by  the  accompanying  martensite  having  been  formed  with 
considerable  dilatation,  (2)  on  reheating  this  martensito-austenitic  steel  the  marten- 
site  is  first  converted  into  troostite,  because  this  transformation  taking  place  with 
contraction  is  readily  induced,  (3)  once  this  transformation  started,  the  pressure  upon 
the  austenite  is  released  and  this  constituent,  in  turn,  passes  first  to  the  martensitic 
stage  (with  increase  of  volume)  and  then  quickly  to  the  troostitic  stage  (with  decrease 
of  volume)  the  martensitic  condition  being  of  so  short  duration  as  to  readily  escape- 
detection. 

Tempering  Martensitic  Steel.  —  It  has  been  shown  that  martensite  is  generally 
present  in  commercially  hardened  steel.  Since  this  constituent  represents  a  partial 
transformation  of  austenite  it  follows  that  it  must  be  more  stable  than  austenite  at 
atmospheric  temperature.  It  is  sufficiently  unstable,  however,  to  be  readily  con- 
verted first  into  troostite  and  then  into  sorbite  on  tempering  as  indicated  in  III, 
Figure  291.  At  400  deg.  the  transformation  of  martensite  into  troostite  is  complete, 
while  at  600  deg.  troostite  is  replaced  by  sorbite.  Bearing  in  mind  the  physical  prop- 
erties of  martensite,  troostite,  and  sorbite,  it  will  be  readily  understood  why,  on  tem- 
pering martensitic  steel,  its  hardness  gradually  decreases  while  it  becomes  less  brittle 
and  indeed  quite  ductile  if  made  sorbitic. 

Tempering  Troostitic  Steel.  —  Commercially  hardened  steel  frequently  contains 
large  proportions  of  troostite.  The  tempering  of  troostite  is  depicted  in  IV,  Figure  291. 
This  constituent  being  decidedly  less  unstable  than  martensite  requires  greater  plas- 
ticity, i.e.  a  higher  temperature,  before  being  transformed  into  a  still  more  stable 
condition.  Experimental  evidences  seem  to  show  that  the  tempering  of  troostite, 
i.e.  its  transformation  into  sorbite,  requires  a  temperature  of  at  least  400  deg.  and 
that  at  600  deg.  the  transformation  is  complete.  Since  in  practise  the  tempering  of 
steel  is  seldom  carried  above  300  deg.  it  would  seem  as  if  steels  made  up  of  troostite 
do  not  need  to  be  tempered,  being  sufficiently  tough.  There  seems  to  be  no  reason 
to  doubt  the  accuracy  of  the  above  inference.  Commercially  hardened  steels,  how- 
ever, are  generally  either  entirely  martensitic  or,  more  frequently,  partly  marten- 
sitic and  partly  troostitic  and  are  in  need  of  tempering  because  of  the  large  propor- 
tion of  the  excessively  hard  and  brittle  martensite  they  usually  contain. 

Tempering  Troostito-Martensitic  Steel.  — •  In  V,  Figure  291,  the  tempering  of  hard- 
ened steel  containing  both  martensite  and  troostite  has  been  depicted.  It  is  assumed 
that  the  martensite  present  begins  to  be  converted  into  troostite  as  soon  as  the  tem- 
perature of  the  metal  rises,  while  the  transformation  of  the  troostite  into  sorbite  begins 
only  at  400  deg. 

Tempering  Troostito-Sorbitic  Steel.  —  From  the  diagram  used  to  illustrate  the 
tempering  of  steel  it  will  be  apparent  that  sorbite  is  relatively  so  stable  a  constituent 


CHAPTER   XVII  — THE   TEMPERING   OF   HARDENED   STEEL  303 

that  its  transformation  into  pearlite  cannot  be  effected  below  the  critical  range  or,  in 
other  words,  that  it  cannot  be  tempered.  Indeed  sorbitic  steels  not  being  hardened 
steels  need  not  be  considered  in  connection  with  the  tempering  operation.  A  graphi- 
cal representation  of  the  tempering  of  troostito-sorbitic  steel,  however,  has  been  in- 
cluded in  Figure  291.  It  shows  that  such  steel  remains  unchanged  until  its' tempera- 
ture reaches  400  deg.  when  the  troostite  it  contains  begins  to  be  transformed  into 
sorbite,  the  transformation  being,  as  usual,  complete  at  600  deg. 

Osmondite.  —  It  has  been  shown  that  on  tempering  hardened _steel  it  is  entirely 
converted  into  troostite  at  about  400  deg.  C.  Below  that  temperature  some  marten- 
site  remains  in  the  structure,  while  above  it  some  sorbite  is  present.  To  the  condition 
of  steel  when  made  up  wholly  of  troostite  Heyn  gives  the  name  of  "osmondite."  It 
will  be  apparent  that  osmondite  does  not  represent  a  new  constituent  but  merely  a 
condition  assumed  by  the  steel  at  a  certain  temperature  and  the  wisdom  of  giving  a 
specific  name  to  that  condition  may  well  be  questioned. 

Troostite  is  more  readily  colored  by  the  usual  etching  reagents  than  any  other 
constituent  of  steel,  from  which  it  follows  that  steel  made  up  exclusively  of  troostite, 
i.e.  in  the  osmondite  condition,  must  exhibit  maximum  coloration.  Again  Heyn  has 
shown  that  the  solubility  of  steel  in  dilute  sulphuric  acid  increases  with  the  amount 
of  troostite  present,  being  maximum  in  steel  tempered  to  about  400  deg.  C.  From 
these  observations  Heyn  described  osmondite  as  a  constituent  of  steel  characterized 
by  maximum  solubility  in  acids  and  by  maximum  coloration  under  the  action  of  acid 
metallographic  reagents.  In  the  report  of  the  Committee  on  the  Nomenclature  of 
the  Constituents  of  Iron  and  Steel  of  the  International  Association  for  Testing  Mate- 
rials osmondite  is  described  as  follows:  "Probably  aggregate.  That  stage  in  the 
transformation  of  austenite  at  which  the  solubility  in  dilute  sulphuric  acid  reaches  its 
maximum  rapidity.  Arbitrarily  taken  as  the  boundary  between  troostite  and  sor- 
bite .  .  .  The  following  hypotheses  have  been  suggested,  none  of  which  has  sub- 
stantial experimental  foundation:  (1)  A  solid  solution  of  carbon  or  an  iron  carbide  in 
alpha  iron;  (2)  The  colloidal  system  of  Benedicks  in  its  purity,  troostite  being  this 
system  while  forming  at  the  expense  of  martensite,  and  sorbite  being  this  system 
coagulating  and  passing  into  pearlite;  (3)  The  stage  of  maximum  purity  of  amorphous 
alpha  iron  in  the  way  to  crystallizing  into  ferrite." 

Structural  Changes  on  Slow  Cooling,  Quick  Cooling,  and  Reheating.  —  It  seems 
helpful  and  instructive  to  depict  graphically  in  a  single  diagram  the  structural  changes 
taking  place  in  eutectoid  steel  (1)  on  slow  cooling  through  its  critical  range,  (2)  on 
quick  cooling  through  that  range,  and  (3)  on  reheating  quickly  cooled  (hardened) 
steel  above  the  range.  The  changes  indicated  in  I  (Fig.  292)  show,  as  already  ex- 
plained, that  steel,  on  cooling  slowly  through  the  critical  range,  is  converted  suc- 
cessively into  martensite,  troostite,  sorbite,  and  pearlite.  On  heating  the  same  steel 
from  below  to  above  the  range  the  same  changes  would  take  place  but  in  the  reverse 
order.  In  II  the  steel  has  been  cooled  through  the  range  at  such  speed  that  marten* 
site  was  formed  but  prevented  from  further  transformation,  hardened  martensitic 
steel  being  produced.  The  reheating  of  this  martensitic  steel  is  depicted  in  III. 
Below  the  range  martensite  is  gradually  converted  first  into  troostite  and  then  into 
sorbite.  On  entering  the  range  the  steel  remains  sorbitic  but  on  further  heating  the 
sorbite  is  converted  back  into  troostite  and  then  into  martensite.  Near  the  top  of  the 
range  austenite  begins  forming,  the  transformation  being  complete  as  the  steel  emerges 
from  its  range.  Similar  structural  transformations  would  take  place  in  subjecting 


304 


CHAPTER  XVII  — THE   TEMPERING   OF   HARDENED   STEEL 


hypo-eutectoicl  and  hyper-eutectoid  steels  to  like  treatments,  but  free  ferrite  or  free 
cementite  would  generally  be  present. 

Microstructure  of  Hardened  and  Tempered  Steel.  —  The  structural  changes 
corresponding  to  the  transformations  taking  place  on  tempering  hardened  steel  de- 
scribed in  the  foregoing  pages  are  not  always  readily  detected  by  microscopical  ex- 
amination. This  is  due  to  the  fact  that,  structurally  speaking,  these  changes  are 
often  pseudomorphic  changes,  the  crystalline  forms  of  the  original  constituent  or 


1000 


800 


•5/ 


Sfeel 


Fig.  292.  —  Diagram  depicting  the  constituents  formed  (I)  on  slow  cooling,  (II)  on  quick  cooling,  and 

(III)  on  reheating  hardened  steel. 

constituents  having  been  retained,  although  the  nature  of  the  crystals  themselves 
has  been  altered. 

Referring  to  pseudomorphism  Dana  writes:  "The  crystalline  forms  under  which  a 
species  occur  are  sometimes  those  of  another  species."  Bayley  defines  pseudomorphs 
as  bodies  possessing  forms  borrowed  from  another  substance,  or  as  a  body  possessing 
the  form  of  one  substance  and  the  chemical  and  physical  properties  of  another.  In 
the  formation  of  a  pseudomorph  the  material  of  the  original  substance  is  replaced 
by  the  new  substance  but  its  external  form  remains  unchanged. 

According  to  Heyn  the  following  appearances  are  observed  in  the  case  of  eutec- 
toid  steel : 

(1)  After  hardening  but  before  tempering:  marlcnsite  with  well-developed  needles 
remaining  uncolored  after  an  immersion  of  10  minutes  in  a  solution  of  one  per  cent  of 
hydrochloric  acid  in  alchohol. 


CHAPTER  XVII  — THE   TEMPERING   OF   HARDENED   STEEL 


305 


(2)  After  tempering  between  100  and  200  cleg. :  martensitic  structure  unchanged 
but  colored  yellow  or  brown. 

(3)  After  tempering  to  275  cleg. :  the  needle  structure  becomes  coarser  and  recalls 
mixtures  of  austenite  and  martensite,  one  of  the  constituents  remaining  uncolored, 
the  other  assuming  a  dark  coloration. 

(4)  After  tempering  to  405  deg. :  the  needles  have  disappeared  and  the  sample 
appears  dark  and  mottled  suggesting  a  mixture  of  two  relatively  dark  constituents; 
this  is  troostite. 

(5)  After  tempering  to  500  deg. :  the  light  areas  become  mofC"  abundant. 

(6)  After  tempering  to  600  deg. :  irregular  meshes  are  observed  rounded  and  light, 
partially  surrounded  by  a  darker  network  which  appear  to  stand  in  relief. 


Fig.  293.  — Steel.  Carbon  0.45  per  cent.  Magnified  1000  diameters. 
Heated  to  825  deg.  C.,  quenched  from  720  deg.,  and  tempered  between 
blue  and  brown  (275  deg.?).  (Osmond.) 


Photomicrographs  of  the  structure  of  hardened  and  tempered  steels  are  repro- 
duced in  Figures  293  and  294.  The  structure  of  hardened  steel  reheated  to  600  deg. 
has  been  shown  in  Figure  231,  Chapter  XV. 

Carbon  Condition  in  Tempered  Steel.  —  It  is  generally  admitted  that  the  com- 
bined carbon  present  in  steel  may  exist  under  two  different  conditions,  (1)  as  harden- 
ing carbon,  that  is,  as  carbon  or  the  carbide  Fe3C  dissolved  in  iron  and  (2)  as  cement 
carbon,  that  is,  as  the  crystallized  carbide  FesC  or  cementite.  It  is  further  believed 
(a)  that  pearlite  is  quite,  if  not  altogether,  free  from  hardening  carbon,  (6)  that  mar- 
tensite contains,  for  a  given  steel,  the  maximum  amount  of  hardening  carbon  that 
can  be  produced  and  retained  in  that  steel  by  the  ordinary  hardening  operation,  and 


306  CHAPTER  XVII  — THE   TEMPERING   OF   HARDENED   STEEL 

(c)  that  on  tempering  martensite  the  amount  of  hardening  carbon  decreases  as  the 
tempering  temperature  increases,  while  the  proportion  of  cement  carbon  increases 
correspondingly.  Heyn,  however,  found  that  on  analyzing  the  residue  remaining  on 
dissolving,  in  dilute  sulphuric  acid,  hardened  eutectoid  steel  tempered  below  400  deg. 
it  contained  no  carbon  in  the  form  of  the  carbide  FesC,  that  is  no  cement  carbon. 
For  the  carbon  remaining  in  the  residue  and  which,  in  his  opinion,  is  different  from 
cement  carbon,  Heyn  suggested  the  notation  Cf.  Not  until  a  temperature  of  400  deg. 
had  been  reached  on  tempering  was  cement  carbon  detected  in  the  residue.  Heyn 
infers  from  these  observations  (1)  that  troostite  contains  no  cement  carbon,  its  resid- 
ual carbon  being  in  the  hypothetical  Cf  condition,  (2)  that  osmondite  contains  the 


Fig.  294.  —  Steel.  Carbon  0.50  per  cent.  Magnified 
500  diameters.  Heated  to  850  deg.  C.,  quenched  in 
water,  reheated  to  400  deg.,  and  quenched  in  water. 
(W.  H.  Knight  in  the  author's  laboratory.) 


maximum  amount  of  Cf  carbon,  and  (3)  that  sorbite  must  be  formed,  that  is,  the  steel 
must  be  tempered  above  400  deg.  in  order  to  produce  cement  carbon.  Heyn  further 
argues  that  the  deep  coloration  produced  on  etching  hardened  and  tempered  steel  is 
caused  by  the  separation  of  Cf  carbon,  being,  therefore,  maximum  in  osmondite,  that 
is,  when  the  steel  contains  nothing  but  troostite. 

Osmond  very  appropriately  remarks  that  it  is  not  necessary  in  order  to  explain 
Heyn's  results  to  admit  the  existence  of  a  new  form  of  carbon,  it  being  quite  possible 
that  the  Cf  carbon  of  Heyn  is  cement  carbon,  that  is,  Fe3C  so  finely  divided  when 
formed  below  400  deg.  that  it  is  readily  decomposed  by  the  acid,  while  above  400  it 
becomes  coarser  and,  therefore,  resists  better  the  action  of  the  acid. 

Decrease  of  Hardness  on  Tempering.  —  According  to  Boynton  the  decrease  of 
hardness  taking  place  on  tempering  is  gradual  up  to  350  deg.,  quite  sudden  between 
350  and  550,  and  nil  above  550  deg. 


CHAPTER   XVII  — THE   TEMPERING   OF   HARDENED   STEEL  307 

Heyn  found  the  following  values  for  the  loss  of  hardness  on  tempering  expressed 
in  per  cent  of  the  original  increase  produced  by  the  hardening  operation : 

at  100  deg.    2.5  per  cent  at  400  deg.  70.0  per  cent 

"  200    "     14.0    "      "  "  500    "    87.5    "       " 

"  300    "     41.0    "       "  "  600     "    97.5    "       " 

Heyn  also  observed  that  the  loss  of  hardness  takes  place  most  quickly  at  300  deg. 

Heat  Liberated  on  Tempering.  —  In  hardening  steel  the  cooling  through  and 
below  its  critical  range  is  so  rapid  that  the  transformation  of  austenite  which  would 
have  taken  place  had  time  been  given  can  proceed  but  partially,  namely,  to  the  mar- 
tensitic  or  troostito-martensitic  stage.  The  heat  which  would  have  been  generated 
had  the  transformation  been  complete  remains  latent  in  hardened  steel.  On  tem- 
pering, however,  the  partially  suppressed  transformation  is  permitted  to  proceed 
farther,  this  return  to  a  more  stable  condition  being  accompanied  by  some  evolution 
of  heat.  According  to  Osmond  this  latent  heat  can  be  made  apparent  by  dissolving 
hardened  steel  in  double  chloride  of  ammonium  and  copper  when  it  evolves  more  heat 
than  unhardened  steel. 

Heyn  made  some  careful  determinations  of  the  heat  generated  on  tempering 
hardened  steel.  The  greater  acceleration  in  heating  hardened  steel  was  made  ap- 
parent by  the  differential  method,  using  as  neutral  bodies  similar  steels  in  their 
pearlitic,  that  is  unhardened,  condition.  It  was  observed  that  the  heat  generated  on 
tempering  is  maximum  at  360  (leg. 

H.  Schottky  reports  that  when  placing  hardened  steel  in  water  vapor  its  tempera- 
ture rises  several  tenths  of  a  degree  above  that  of  the  vapor,  from  which  it  must  be 
inferred  that  even  at  so  low  a  temperature  some  tempering  must  take  place.  He 
finds  that  the  evolution  of  heat  increases  with  the  carbon  content  up  to  a  certain 
maximum  and  then  decreases.  The  following  results  were  obtained. 

CARBON  QUENCHING  TEMPERATURE 

PER  CENT  TEMP.  DEG.  C.  OF  STEEL  PIECES 

0.54  1000  100.15 

0.89  1100  100.35 

1.22  1000  100.55 

1.43  1300  100.33 

1.10  1350  100.20 


CHAPTER   XVIII 


THEORIES   OF  THE  HARDENING   OF  STEEL 

Many  theories  have  been  put  forward  to  explain  the  hardening  of  steel  through 
sudden  cooling  from  a  high  temperature.  They  may  be  divided  into  two  classes, 
(I)  the  "retention"  theories  and  (II)  the  stress  theories.  The  retention  theories  in- 
clude (A)  the  solution  theories,  (5)  the  amorphous  iron  theory,  and  (C)  the  carbon 
theories.  Two  solution  theories  at  least  have  been  proposed,  (a)  the  beta  iron  theory 
and  (6)  the  alpha  iron  theory,  while  two  carbon  theories  should  be  mentioned,  (a)  the 
hardening  carbon  theory  and  (6)  the  subcarbide  theory.  The  stress  theories  include 
(A)  the  early  theory,  (B)  the  interstrain  theory,  and  (C)  the  twinning  and  amorphous 
iron  theory.  This  classification  of  the  hardening  theories  is  given  below  in  a  tabular 
form,  as  well  as  the  names  of  their  proposers. 


(I)  Retention 
theories 


(A)  Solution 
theories 


(II)  Stress 

theories 


(a)  Beta  iron  or  Osmond 

allotropic  theory 

(6)  Alpha  iron  theory     H.  Le  Chatelier 
(B)  Amorphous  iron  theory  Humfrey 

C   (a)  Hardening  carbon 

theory 

I   (6)  Subcarbide  theory 
(A)  Early  stress  theory 


(C) 


Carbon 
theories 


(5)  Interstrain  theory 
(C)  Twinning  and  amorphous 
iron  theory 


Arnold 

Andre   Le  Chate- 
lier 

McCance 

Carpenter    and 
Edwards 


Retention  Theories.  —  The  retention  theories  claim  that  in  hardening  steel  a  con- 
dition or  set  of  conditions  existing  normally  above  its  critical  range  is  retained  un- 
changed in  the  cold  or  but  partially  changed  because  of  the  rapid  cooling  through  and 
below  the  critical  range.  In  other  words,  such  very  quick  cooling  through  the  range 
denies  the  necessary  time  for  the  transformations  to  take  place,  at  least  fully,  and  a 
condition  is  preserved  in  the  cold  which  is  stable  only  above  or  within  the  critical 
range.  According  to  these  theories  hardened  steel,  therefore,  is  in  an  unstable  condi- 
tion, hence  the  possibility  of  tempering  it.  That  the  transformations  which  would 
have  taken  place  on  slow  cooling  through  the  range  are  suppressed,  partly  at  least,  in 
hardening  is  made  evident  by  the  absence  of  critical  points  during  very  rapid  cooling 
or  by  the  appearance  of  feeble  points  only  at  greatly  lowered  temperatures.  Other 
evidences  of  the  partial  suppression  of  the  transformations  are  afforded  (1)  by  the 
condition  of  the  carbon  in  hardened  steel  which  is  different  from  the  condition  of 
that  element  in  unhardened  steel,  (2)  by  the  evolution  of  heat  taking  place  on  tem- 

308 


CHAPTER  XVIII  —  THEORIES   OF   THE   HARDENING   OF   STEEL  309 

pering  hardened  steel  as  explained  in  Chapter  XVII,  and  (3)  by  the  structure  of 
hardened  steel. 

To  assume  that  hardened  steel  owes  its  hardness  to  the  suppression,  partial  at 
least,  of  the  transformations  taking  place  on  slow  cooling  through  the  critical  range 
is,  therefore,  both  natural  and  logical. 

When  we  come  to  look  into  the  nature  of  the  constituents  stable  above  or  within 
the  range,  which  on  being  retained  in  the  cold  impart  extreme  hardness  to  steel,  we 
find  very  great  differences  of  opinion  among  competent  authorities. 

Solution  Theories., —  Many  writers  believe  that  hardened  steel  is  in*  the  condi- 
tion of  a  solid  solution,  quick  cooling  through  the  range  having  prevented  the  forma- 
tion of  the  ferrite-cementite  aggregate.  This  is  strongly  supported  by  microscopical 
and  other  evidences.  They  all  agree  that  the  solution  consists  of  carbon  dissolved 
in  iron  but  different  views  are  held  in  regard  to  the  condition  of  the  carbon  and  of 
the  iron.  While  it  is  now  the  general  belief  that  the  carbide  FesC  rather  than  ele- 
mentary carbon  is  dissolved  in  iron,  some  think  that  in  hardened  steel  the  iron  is 
chiefly  present  as  beta  iron,  while  others  believe  that  it  exists  mainly  in  the  alpha 
condition. 

Beta  Iron  or  Allotropic  Theory.  —  This  theory  put  forward  with  great,  vigor  and 
brilliancy  by  Osmond,  championed  first  by  the  late  Roberts-Austen  and  later  by 
Howe  and  many  other  eminent  metallurgists,  contends  that  in  hardened  steel  iron  is 
present  chiefly  in  the  beta  condition.  It  is  often  referred  to  as  the  allotropic  theory 
of  the  hardening  of  steel.  It  should  be  borne  in  mind,  however,  that  Osmond's  allo- 
tropic theory  of  iron  and  his  allotropic  theory  of  the  hardening  of  steel  are  two  differ- 
ent conceptions.  One  may  believe  in  the  former  without  being  an  adherent  of  the 
latter.  The  allotropic  theory  of  iron  claims  that  iron  exists  in  at  least  two  and  prob- 
ably in  three  allotropic  forms.  No  one  doubts  the  allotropy  of  iron  although  some 
writers  believe  that  only  two  allotropic  forms,  namely  gamma  and  alpha  iron,  have 
been  shown  to  exist.  The  allotropic  theory  of  the  hardening  of  steel  claims  (1)  that 
in  hardened  steel  carbon,  or  more  probably  the  carbide  Fe3C,  is  in  solution  chiefly 
in  beta  iron,  hence  its  hardness,  beta  iron  being  very  hard,  (2)  that  hardened  steel 
contains  alpha  iron,  hence  its  magnetism,  alpha  iron  being  the  only  allotropic  form 
under  which  iron  is  magnetic. 

While  the  allotropists  regard  the  retention  of  beta  iron  as  the  chief  cause  of  the 
hardening  of  steel,  they  do  not  ignore  the  very  important  part  played  by  carbon. 
They  realize  that  the  presence  of  carbon  is  essential  to  the  retention  of  iron  in  its  hard 
allotropic  form,  for  in  the  absence  of  carbon  it  is  not  possible  to  harden  iron.  It  is 
customary  to  compare  this  action  of  carbon  in  preventing  the  transformations  to  that 
of  a  brake,  the  more  carbon  the  more  powerful  the  brake  action,  hence  the  harder  the 
steel  because  of  the  retention  of  a  larger  quantity  of  beta  iron.  They  believe,  however, 
that  the  hardness  of  quenched  steel  increases  with  the  proportion  of  the  beta  iron  it 
contains  rather  than  with  the  proportion  of  carbon.  In  other  words,  that  if  it  were 
possible  to  retain  the  same  amount  of  beta  iron  with  less  carbon  or  even  in  the  com- 
plete absence  of  carbon  the  metal  would  be  equally  hard.  The  allotropists  do  not 
claim  that  steels  hardened  in  the  ordinary  way,  that  is,  martensitic  steels,  are  abso- 
lutely free  from  gamma  iron.  Evidences  are  lacking  to  settle  this  point.  Again  the 
allotropists  do  not  deny  that  the  internal  pressure  created  by  the  transformation  of 
austenite  into  the  more  bulky  martensite  may  contribute  to  the  hardness  of  the 
metal. 


310  CHAPTER  XVIII  —  THEORIES   OF   THE   HARDENING   OF   STEEL 

Summing  up,  in  the  light  of  this  theory,  the  hardening  of  steel  by  rapid  cooling  is 
thus  explained:  (1)  the  bulk  of  the  iron  passes  from  the  gamma  to  the  beta  condi- 
tion, hence  the  great  hardness  produced,  (2)  some  of  the  beta  iron  is  further  trans- 
formed into  alpha  iron,  hence  the  magnetism  of  hardened  steel,  (3)  a  large  proportion 
of  the  carbon  or  more  probably  of  the  carbide  Fe3C  remains  dissolved  in  the  beta 
iron,  the  presence  of  this  dissolved  (hardening)  carbon  in  hardened  steel  being  proven 
by  chemical  analysis,  (4)  the  internal  pressure  created  by  the  transformation  of  aus- 
tenite  into  martensite,  that  is,  of  gamma  into  beta  iron,  may  contribute  to  the  final 
hardness.  Osmond's  theory  of  the  hardening  is,  therefore,  based  on  the  belief  (1)  in 
the  existence  of  beta  iron  and  (2)  in  the  hardness  of  beta  iron. 

On  tempering  hardened  steel  Heyn  found  that  70  per  cent  of  the  increased  hard- 
ness produced  by  the  hardening  operation  were  lost  in  tempering  below  400  deg., 
the  remaining  30  per  cent  being  possibly  due,  according  to  Osmond,  to  the  internal 
pressure  already  alluded  to.  He  further  observed  that  hardening  (dissolved)  carbon 
did  not  begin  to  be  converted  into  cement  carbon  (crystallized  Fe3C)  until  a  tempera- 
ture of  400  deg.  was  reached.  Steel  then  loses  most  of  its  hardness  while  its  carbon 
remains  in  the  hardening  condition.  From  this  coexistence  of  softness  and  harden- 
ing carbon  it  logically  follows  that  steel  dees  not  owe  its  hardness  to  the  presence  of 
hardening  carbon,  and  that  the  presence  of  allotropic  beta  iron  remains  the  only 
possible  explanation.  This  conclusion  is  further  supported  by  the  fact  that  on  tem- 
pering steel  it  is  chiefly  below  400  deg.  that  heat  is  liberated  and  this  liberation  must 
necessarily  be  ascribed  to  the  iron  returning  from  the  beta  to  the  alpha  condition. 

Alpha  Iron  Theory.  —  Le  Chatelier  and  Guillet  believe  that  on  quick  cooling 
through  the  critical  range  the  allotropic  transformation  of  iron  from  its  gamma  to 
its  alpha  condition  is  not  prevented  but  that  the  steel  remains,  nevertheless,  in  the 
condition  of  a  solid  solution,  hardened  steel  in  their  opinion  being  a  solid  solution  of 
carbon  (or  of  the  carbide  Fe3C)  in  alpha  iron  owing  its  hardness  to  its  state  of  solu- 
tion and  its  magnetism  to  the  presence  of  alpha  iron.  This  view  is  based  chiefly 
upon  these  writers'  belief  that  the  point  A3  is  not  an  allotropic  point  and  that  there- 
fore beta  iron  does  not  exist. 

Amorphous  Iron  Theory.  —  According  to  J.  C.  W.  Humfrey:  "The  hard  struc- 
ture which  can  be  produced  in  carbon  steels  by  quenching  and  in  certain  alloy  steels 
by  normal  cooling,  is  due  to  the  presence  of  a  hard  amorphous  solution  of  a  iron  and 
iron  carbide,  which  solution  may  be  compared  to  Beilby's  amorphous  phase  formed 
by  overstrain. 

"To  explain  the  formation  of  this  amorphous  phase  the  author  advances  a  theory 
that  the  passage  of  a  substance  from  one  allotropic  modification  to  another  of  differ- 
ent crystalline  form  involves  the  temporary  formation  of  an  amorphous  state,  corre- 
sponding to  the  liquid  phase  of  the  modification  about  to  be  formed.  In  steels  such 
a  change  occurs  at  Ar3;  and  if,  due  to  sudden  cooling  or  to  the  presence  of  certain 
alloyed  elements,  the  change-point  is  lowered  to  a  temperature  below  that  at  which 
crystallization  in  the  viscous  mass  becomes  difficult,  then  the  amorphous  form  will 
be  retained  in  a  metastable  form  in  the  cold." 

Carbon  Theories.  —  The  carbon  theories  contend  that  the  hardness  of  rapidly 
cooled  steel  is  due  primarily  to  the  retentibn  in  the  cold  of  a  very  hard  condition  of 
the  carbon  normally  stable  only  above  the  range,  the  allotropic  transformation  of 
iron  playing  no,  or  but  an  unimportant,  part  in  the  phenomenon.  As  supporting 
their  claims  they  point  to  the  fact  that  carbonless  iron  cannot  be  hardened  and  that 


CHAPTER  XVIII  —  THEORIES   OF   THE   HARDENING   OF   STEEL  311 

the  more  carbon  present  the  greater  the  increased  hardness  produced  by  quick  cool- 
ing, at  least  up  to  the  eutectoid  carbon  ratio. 

These  theories  differ  in  regard  to  the  exact  condition  of  the  carbon  thus  retained 
by  quenching  and  imparting  great  hardness  to  the  metal. 

The  Hardening  Carbon  Theory.  —  It  was  held  for  many  years  by  the  majority 
of  writers  that  hardened  steel  owed  its  hardness  to  the  presence  of  hardening  carbon, 
a  form  of  carbon  stable  only  above  the  range  but  which  could  be  retained,  in  part  at 
least,  by  quick  cooling.  This  belief  rested  on  the  apparent  difference  existing  between 
the  condition  of  the  carbon  in  hardened  and  in  unhardened  steels  as  proven  by  dis- 
solving them  in  cold  dilute  acids  when  a  large  proportion  of  the  carbon  of  hardened 
steel  escapes  as  hydrocarbons,  whereas  nearly  the  totality  of  the  carbon  of  unhard- 
ened steel  remains  as  a  residue  which,  upon  being  analyzed,  is  found  to  consist  of  the 
carbide  FesC.  As  to  the  exact  nature  of  hardening  carbon,  vague,  conflicting,  and 
often  extraordinary  statements  appeared,  it  being  claimed  by  some,  for  instance,  that 
hardening  carbon  was  carbon  in  a  diamond-like  condition.  It  is  at  present  believed 
by  most  that  hardening  carbon  is  carbon  (or  more  probably  the  carbide  FeaC)  dis- 
solved'in  iron,  its  escape  as  hydrocarbons  upon  being  subjected  to  the  action  of  dilute 
acids  being  due  to  its  extremely  fine  state  of  division.  If  this  bo  the  nature  of  harden- 
ing carbon,  the  hardening  carbon  theory  becomes,  of  course,  a  solution  theory. 

It  is  obvious  that  carbon  as  such,  no  matter  how  great  its  hardness,  could  not 
impart  extreme  hardness  to  steel  in  which  it  may  be  associated  with  199  times  its 
weight -of  soft  ferrite  as,  for  instance,  in  steel  containing  0.50  per  cent  carbon.  The 
contention  that  it  is  not  carbon,  as  such,  which  is  retained  by  quick  cooling  but  a  very 
hard  carbide  constituting  the  whole  or  a  large  part  of  hardened  steel  is  not,  of  course, 
open  to  the  same  objections.  This  is  the  claim  of  the  subcarbide  theory. 

The  Subcarbide  Theory.  —  Arnold  contends  that  eutectoid  steel  above  its  critical 
range  exists  as  the  carbide  Fe24C,  a  chemical  compound  containing  about  0.89  per 
cent  carbon.  This  carbide  which  he  calls  "hardenite"  is  very  hard  and  being  re- 
tained by  quick  cooling  imparts  hardness  to  quenched  steel.  In  hypo-eutectoid  steel 
some  ferrite  and  in  hyper-eutectoid  steel  some  cementite  are  dissolved  in  this  sub- 
carbide.  It  follows  from  this  theory  that  austenite  and  martensite  correspond  to 
different  structural  appearances  of  the  same  constituent,  namely,  the  carbide  Fe24C 
when  of  eutectoid  composition,  the  same  carbide  plus  ferrite  or  cementite  in  hypo-  or 
hyper-eutectoid  steel.  This  theory  is  purely  of  a  speculative  character,  the  existence 
of  the  carbide  Fe24C  not  being  supported  by  a  single  direct  evidence.  It  is,  moreover, 
strongly  opposed  by  the  universally  accepted  theory  of  metallic  alloys  which  holds 
that  eutectic  (and  eutectoid)  alloys  immediately  before  their  formation  are  not  defi- 
nite chemical  compounds  but  liquid  or  solid  solutions.  On  forming,  whether  or  not 
it  implies  a  change  of  state,  the  solution  is  transformed  into  an  aggregate  of  the 
solute  and  solvent,  ferrite  and  cementite  in  the  case  of  iron-carbon  alloys.  The 
breaking  up  at  a  certain  critical  temperature  of  a  definite  chemical  compound,  as 
demanded  by  Arnold's  theory,  into  a  eutectoid  aggregate  of  the  elements  of  that 
compound  is  contrary  to  our  firmly  established  knowledge  of  the  mechanism  of  the 
formation  of  such  aggregates.  It  implies  a  return  to  Guthrie's  original  error. 

The  Stress  Theories.  —  In  cooling  steel  quickly  from  above  its  critical  range  it  is 
subjected  to  two  kinds  of  stresses,  (1)  stresses  due  to  the  shrinkage  of  its  outer  shell 
on  its  interior  and  (2)  stresses  due  to  the  transformation  with  increased  volume  of 
gamma  into  beta  and  alpha  iron.  The  existence  of  the  strains  resulting  from  these 


312  CHAPTER   XVIII  —  THEORIES   OF   THE   HARDENING    OF   STEEL 

stresses  have  been  claimed  to  account  satisfactorily  for  the  hardening  of  steel  by 
sudden  cooling.  It  was  argued  long  ago,  for  instance,  that  the  hardening  of  steel  by 
sudden  cooling  might  be  due  to  the  metal  being  in  a  severely  strained  condition,  be- 
cause of  the  quicker  cooling  of  the  outer  layers,  these  layers  through  their  contrac- 
tion exerting  a  severe  pressure  upon  the  central  portion  of  the  steel  objects.  The 
advocates  of  this  theory  pointed  to  the  increased  hardness  resulting  from  cold-work- 
ing steel  as  a  proof  that  severe  straining  produces  hardness.  Some  went  as  far  as  to 
claim  that  cold  worked  steel  and  hardened  steel  are  practically  in  the  same  physical 
condition,  the  quenching  of  steel  producing,  so  to  speak,  an  internal  cold-working 
(straining)  of  the  metal.  They  seem  to  have  overlooked  the  enormous  difference  be- 
tween the  relatively  small  increase  of  hardness  produced  by  cold-working  and  the 
hardness  resulting  from  quenching.  In  the  fact  that  both  cold-working  and  harden- 
ing increase  the  elastic  limit  and  decrease  the  ductility  they  found  additional  support 
for  their  view.  According  to  Osmond  the  belief  once  held  that  cold-working  causes 
an  allotropic  transformation  is  now  abandoned.  While  it  is  not  unreasonable  to  as- 
sume that  the  strained  condition  of  the  metal  adds  to  its  hardness  it  is  hardly  think- 
able that  the  sudden  and  very  great  increase  of  hardness  produced  by  quick  cooling 
is  due  altogether  to  this  straining.  If  it  were  so  the  outer  layers  of  quenched  steel 
implements  should  not  be  hard  and  razor  blades  could  not  be  hardened.  Again,  it  is 
impossible  to  reconcile  this  theory  with  the  fact  that  it  is  necessary  to  quench  steel 
from  above  its  critical  range  in  order  to  harden  it,  for  one  cannot  conceive  why,  if  the 
steel  be  quenched  slightly  below  the  range,  the  strain  created  in  the  quenching  bath 
would  be  so  slight  as  to  have  no  hardening  effect,  whereas  quenching  from  a  tempera- 
ture but  a  few  degrees  higher  would  produce  very  severe  straining.  Finally,  if  the 
contraction  of  the  outer  layers  due  to  their  rapid  cooling  can  induce  such  hardening 
strains  in  the  case  of  steel,  it  is  surprising  that  a  similar  phenomenon  is  not  observed 
in  the  case  of  other  metals.  The  internal  strains  resulting  from  the  transformation 
of  gamma  into  beta  iron  (austenite  into  martensite)  with  increased  volume  afford  a 
more  acceptable  explanation  of  the  hardening  of  steel.  It  was  offered  long  ago  and 
has  recently  been  revived  and  presented  in  a  more  scientific  way,  notably  by  Andre" 
Le  Chatelier,  Charpy,  and  Grenet.  It  is  argued  that  on  quick  cooling  the  allotropic 
transformations  take  place,  partially  at  least,  at  a  temperature  so  low  that  the  metal 
lacks  the  necessary  plasticity  to  yield  to  the  severe  stress  excited  by  these  transforma- 
tions, remaining,  therefore,  severely  strained.  It  becomes  internally  "ecroui,"  as  the 
French  express  it.  Here  again,  however,  it  would  seem  as  if  the  outer  layers  should 
not  be  strained  and  should,  therefore,  remain  soft,  which  of  course  is  contrary  to 
facts.  Nor  is  the  failure  of  carbonless  iron  to  harden  satisfactorily  explained  by  this 
theory. 

Grenet  believes  that  in  hardened  steel  the  allotropic  transformations  are  complete, 
that  is,  that  its  iron  exists  only  in  the  alpha  condition  but  so  severely  strained  (ecroui) 
as  to  be  very  hard.  He  rests  his  belief  chiefly  on  his  assertion  that  on  quick  cooling 
the  dilatations  indicative  of  the  allotropic  transformations  are  not  suppressed  and 
that  the  metal  does  not  remain  non-magnetic.  He  overlooks  the  claims  of  the  advo- 
cates of  the  retention  theories,  so  strongly  supported,  that  the  transformations  are 
not  completely  suppressed,  hence  the  occurrence  of  'a  dilatation  and  of  magnetism. 
When  they  are  completely  prevented,  as  in  austenitic  steels,  the  metal  neither  expands 
nor  becomes  magnetic  on  cooling. 

The  liberation  of  heat  observed  on  tempering  hardened  steel  points  to  a  return  i<> 


CHAPTER  XVIII  —  THEORIES   OF  THE   HARDENING   OF   STEEL  313 

a  more  stable  condition,  supporting,  therefore,  the  retention  theories  and  opposing 
the  stress  theories. 

Interstrain  Theory.  —  Andrew  McCance  believes  that  on  cooling  steel  quickly 
from  above  its  critical  range  the  whole  of  the  carbon  remains  in  solution  while  the 
bulk  of  the  gamma  iron  is  converted  into  alpha  iron.  This  alpha  iron  however  is  in 
an  interstrained  condition  owing  to  the  fact  that  its  crystalline  units  have  been  denied 
the  time  to  assume  an  homogeneous  orientation  and  the  hardness  of  quenched  steel 
is  due  to  this  interstrained  condition.  It  is  further  contended  that  strained  iron  can- 
not be  described  as  amorphous  because  if  it  were  amorphous  it  could  not  be  ferro- 
magnetic, interstrain  in  his  opinion  being  a  better  term  to  describe  its  condition. 
McCance  writes:  "On  quenching  steel,  the  carbon  is  retained  in  solution  and  in  turn 
it  retains  a  proportion  of  the  iron  in  the  gamma  condition,  but  the  majority  of  the 
iron  is  alpha  iron.  At  the  quenching  temperature  the  crystal  grains  had  the  crystal- 
line symmetry  of  gamma  iron,  and  the  gamma  iron  retained  in  the  quenched  state 
by  the  carbon  will  form  an  internal  network  in  these  original  steel  grains.  The  rest 
of  the  iron  will  be  transformed,  and  will  form  crystal  units  of  alpha  symmetry,  but 
these  will  be  prevented  by  lack  of  time,  by  internal  friction,  and  by  the  gamma  iron 
network,  from  arranging  themselves  to  form  homogeneously  oriented  alpha  iron 
crystals.  The  alpha  iron  will  be  in  a  condition  similar  to  interstrain,  and  great  hardness 
trill  result." 

Twinning  and  Amorphous  Iron  Theory.  —  Carpenter  and  Edwards  argue  that 
in  quenching  steel  very  severe  internal  stresses  are  set  up  causing  internal  straining 
of  the  metal  which  in  turn  results  in  the  formation  of  numerous  twins  and  of  hard 
amorphous  layers.  The  markings  of  martensite  in  their  opinion  correspond  to  these 
twins.  Edwards  writes:  "That  the  material  is  internally  strained  is  evident  from  the 
facts  which  have  been  published,  namely,  the  metallic  crystals  are  broken  up  into 
an  exceedingly  large  number  of  twin  lamellae.  Further  we  believe  that  the  hardness 
produced  by  quenching  is  brought  about  by  crystal  twinning  and  possibly  direct 
slipping  and  the  formation  of  amorphous  layers  as  a  result  of  the  internal  deformation." 

Tempering  and  the  Retention  Theories.  —  The  tempering  of  hardened  steel,  as 
already  explained,  is  readily  accounted  for  by  the  retention  theories  on  the  ground 
that  the  metal  being  in  an  unstable  condition  is  ever  eager  to  assume  a  more  stable 
form,  implying  a  return,  partial  at  least,  of  the  iron  to  the  alpha  condition  and  of  the 
carbon  to  the  cement  condition.  On  heating  the  steel  but  slightly  above  atmospheric 
temperature  its  rigidity  is  sufficiently  diminished  to  permit  a  slight  transformation 
of  this  kind,  the  higher  the  temperature  the  more  pronounced  of  course  being  its 
tempering. 

Tempering  and  the  Stress  Theory.  —  The  stress  theory,  likewise,  satisfactorily 
accounts  for  the  tempering  of  hardened  steel  on  the  ground  that  upon  slight  reheat- 
ing the  internal  strains  are  sufficiently  released  to  produce  an  appreciable  decrease  of 
the  specific  effects  of  hardening,  namely,  decrease  of  hardness,  of  strength,  of  elastic 
limit  and  increased  ductility. 

Summary.  —  It  seems  quite  possible,  even  probable,  that  the  various  theories, 
while  apparently  antagonistic,  bring  each  their  contribution  to  the  elucidation  of  the 
problem.  Should  we  not  believe  with  the  allotropists  that  the  hardness  of  steel  is  due 
chiefly  to  the  retention  of  a  large  quantity  of  a  hard  allotropic  variety  of  iron,  possi- 
bly amorphous  and  of  the  dissolved  or  hardening  carbon  of  the  carbonists.  Should 
we  not  with  the  advocates  of  the  stress  theories  believe  in  the  hardening  influence  of 


314  CHAPTER   XVIII  —  THEORIES   OF   THE   HARDENING   OF   STEEL 

the  strains  created  on  quick  cooling  (a)  because  of  the  shrinkage  of  the  outer  layers 
of  the  metal  and  (6)  because  of  the  expansion  accompanying  the  transformation  of 
gamma  into  beta  iron?  None  of  these  theories  alone  gives  a  fully  satisfactory  ex- 
planation: Beta  iron  cannot  be  retained  in  the  absence  of  carbon  and  if  it  could  be 
it  is  not  certain  that  it  would  be  intensely  hard;  the  presence  of  intensely  hard  car- 
bon or  iron  carbide  as  the  chief  cause  of  hardening  is  contrary  to  evidences;  the 
strained  condition  of  hardened  steel  alone  does  not  account  satisfactorily  for  its  ex- 
treme hardness;  Le  Chatelier's  contention  that  quickly  cooled  steel  is  hard  although 
its  iron  is  in  the  soft  alpha  condition  because  of  its  being  in  a  state  of  solution  is  open 
to  objection;  Arnold's  theory  that  hardened  steel  owes  its  hardness  to  the  retention 
of  a  hard  subcarbide  of  iron  lacks  experimental  support  and  is  scientifically  un- 
tenable. 

The  author  concluded  as  follows  a  recent  paper  on  "Metallography  and  the 
Hardening  of  Steel"  presented  at  the  International  Engineering  Congress  in  Sep- 
tember, 1915,  in  San  Francisco: 

"It  will  be  obvious  from  the  foregoing  that  the  many  recent  attempts  at  arriving 
at  a  satisfactory  explanation  of  the  hardening  of  steel  are  based  on  one  or  more  of  the 
following  conceptions:  (1)  existence  of  a  hard  allotropic  variety  of  iron,  (2)  existence 
of  solid  solutions  involving  the  occurrence  of  so-called  "hardening"  carbon,  and 
(3)  existence  of  strains  in  quenched  steel  causing  or  not  an  amorphous  condition  of 
the  iron. 

"It  will  likewise  be  obvious  that  no  theory  so  far  presented  fully  satisfies  our 
craving  for  a  scientifically  acceptable  explanation  of  the  many  phenomena  involved. 

"It  would  seem  as  if  the  methods  used  to  date  for  the  elucidation  of  this  complex- 
problem  have  yielded  all  they  are  capable  of  yielding  and  that  further  straining  of 
these  methods  will  only  serve  to  confuse  the  issue,  a  point  having  been  reached  when 
this  juggling,  no  matter  how  skilfully  done,  with  allotropy,  solid  solutions,  and  strains 
is  causing  weariness  without  advancing  the  solution  of  the  problem.  The  tendency 
of  late  has  been  to  abandon  the  safer  road  of  experimental  facts  and  to  enter  the  maze 
of  excessive  speculations,  in  which  there  is  great  danger  of  some  becoming  hopelessly 
lost. 

"The  conclusion  seems  warranted  that  new  avenues  of  approach  must  be  found  if 
we  are  ever  to  obtain  a  correct  answer  to  this  apparent  enigma." 


CHAPTER  XIX 

THE  CEMENTATION  AND   CASE  HARDENING   OF  STEEL 

The  affinity  of  iron  for  carbon  is  so  great  that  when  heated  to  a  sufficiently  high 
temperature  in  contact  with  some  suitable  carbonaceous  matter  it  readily  absorbs 
carbon.  If  the  heating  be  protracted  (several  days)  and  the  amount  of  carbon  ab- 
sorbed considerable,  the  operation  is  known  as  "cementation"  and  the  resulting 
metal  as  "cemented,"  "converted,"  or  "blister"  steel,  or  in  Sheffield,  England,  as 
"blister  bar,"  while  if  the  treatment  be  of  relatively  short  duration  (a  few  hours)  and 
the  absorption  of  carbon  in  consequence  superficial,  it  is  called  "case  hardening." 

Cementation  is  generally  applied  to  wrought-iron  bars  which  are  afterwards 
melted  (crucible  process)  and  shaped  into  finished  articles  by  casting  or  forging, 
while  case  hardening  is  applied  directly  to  finished  objects  generally  of  low  carbon 
steel.  The  purpose  of  cementation  is  to  introduce  carbon  into  wrought  iron,  thereby 
converting  it  into  steel,  the  subsequent  treatments  (melting,  forging)  producing  a 
uniform  distribution  of  the  carbon,  whereas  the  purpose  of  case  hardening  is  to  man- 
ufacture steel  objects  with  hard  skins  or  cases  while  retaining  their  soft  and  tough 
centers  or  cores. 

The  quantity  of  carbon  thus  absorbed  by  iron  at  a  high  temperature  but  below  its 
melting-point  depends  chiefly  upon  (1)  the  composition  of  the  iron  or  steel  subjected 
to  carburizing,  (2)  the  carburizing  temperature,  (3)  the  length  of  time  at  that  tem- 
perature, and  (4)  the  nature  of  the  carburizing  material. 

Composition  of  the  Iron  or  Steel  Subjected  to  Carburizing.  —  It  is  probably  true 
that  the  smaller  the  proportion  of  carbon  in  the  iron  the  more  eagerly  will  it  take  up 
carbon,  from  which  it  follows  that  as  the  carburizing  proceeds,  that  is,  as  the  metal 
becomes  more  highly  carburized,  additional  introduction  of  carbon  requires  progres- 
sively longer  time,  the  metal  acting  in  this  way  not  unlike  a  solution  approaching  its 
saturation  point. 

In  the  cementation  process  bars  of  very  pure  wrought  iron  and  in  "case  harden- 
ing" steel  objects  containing  at  the  most  0.20  per  cent  carbon  are  subjected  to  the 
carburizing  treatment.  The  steel  should  not  generally  contain  over  0.40  per  cent  of 
manganese  lest  the  case  be  too  brittle.  The  presence  of  certain  elements  appear  to 
hinder  the  carburizing  operation  while  others  facilitate  it. 

According  to  Guillet  the  absorption  of  carbon  is  favored  by  those  special  elements 
which  exist  as  double  carbides  such  as  manganese,  tungsten,  chromium,  molybdenum, 
and  opposed  by  those  which  form  solid  solutions  with  iron  such  as  nickel,  silicon,  and 
aluminum. 

Carburizing  Temperature.  —  While  it  has  been  claimed  that  iron  below  its  critical 
range  will  absorb  some  carbon  this  absorption,  if  taking  place  at  all,  is  very  slow, 
from  which  it  is  logical  to  infer  that  alpha  iron  has  very  little,  if  any,  dissolving'power 

315 


316     CHAPTER  XIX  — THE  CEMENTATION  AND  CASE  HARDENING  OF  STEEL 

for  carbon.  In  order  to  produce  quick  and  intense  carburization  the  iron  should  be 
in  its  beta  or,  more  probably,  in  its  gamma  condition,  and  steel,  therefore,  in  the  con- 
dition of  a  solid  solution.  Cementing  and  case  hardening  operations  must  conse- 
quently be  conducted  above  the  critical  range  of  the  iron  or  low  carbon  steel  treated, 
that  is,  at  a  temperature  exceeding  825  deg.  C.  It  is  also  certain  that  the  higher  the 
temperature  the  quicker  will  carbon  be  absorbed  and  the  deeper  will  it  penetrate  into 
the  steel,  that  is,  the  deeper  the  "case."  At  Sheffield,  England,  where  the  cementa- 
tion process  is  used  more  extensively  than  anywhere  else  the  carburizing  tempera- 
ture is  in  the  vicintiy  of  950  to  1000  deg.  Most  case  hardening  treatments  are  prob- 
ably conducted  in  the  vicinity  of  900  to  950  deg.  C. 

Time  at  Carburizing  Temperature.  —  The  amount  of  carbon  absorbed,  and  there- 
fore the  thickness  of  the  case  as  well,  increases,  of  course,  with  the  length  of  the 


Fig.  295.  —  Steel.     Case  hardened.     Magnified  20  diameters. 

operation  but,  as  already  mentioned,  carburization  takes  place  more  and  more  slowly 
as  the  carbon  content  increases.  The  maximum  amount  of  carbon  which  iron  can 
take  up  while  in  the  solid  state  is  probably  not  far  from  2.50  per  cent,  this,  however, 
requiring  a  protracted  treatment  at  a  very  high  temperature.  While  in  the  manu- 
facture of  blister  steel  considerably  more  than  one  per  cent  of  carbon  is  frequently 
introduced  into  the  wrought-iron  bars,  in  carburizing  finished  steel  articles  it  is  seldom 
desired  to  produce  a  case  containing  more  than  one  per  cent  of  carbon  near  the  out- 
side, a  superficial,  carburized  layer  of  eutectoid  composition  (0.85  per  cent  C.)  being 
generally  considered  to  yield  the  best  results.  The  length  of  time  needed  to  produce 
the  desired  degree  of  carburization  and  desired  depth  of  case  must  necessarily  de- 
pend upon  the  nature  of  the  metal,  the  kind  of  carburizing  material  used,  and  the 
temperature. 

Distribution  of  the  Carbon.  —  It  will  be  apparent  from  the  nature  of  the  opera- 
tion that  in  this  carburizing  of  solid  iron  carbon  travels  slowly  from  the  outside 
towards  the  center  and  that,  therefore,  the  proportion  of  carbon  absorbed  must 


CHAPTER  XIX  — THE  CEMENTATION  AND  CASE  HARDENING  OF  STEEL      317 

decrease  from  outside  to  center,  unless  indeed  the  objects  treated  are  very  thin  or 
the  treatment  so  long  and  conducted  at  so  high  a  temperature  as  to  cause  even  the 
center  to  absorb  the  maximum  amount  of  carbon.  The  decrease  of  carbon  as  one  ap- 
proaches the  core  of  the  object  is  well  illustrated  in  Figures  295  and  296.  A  band  of 
hyper-eutectoid  steel  characterized  by  the  presence  of  free  cementite  is  frequently 
noted  (Fig.  295)  followed  by  a  band  of  eutectoid  composition  characterized  by  the 
absence  of  both  free  cementite  and  free  ferrite  and  this  in  turn  is  followed  by  a  band 
showing  abrupt  and  rapid  decrease  of  carbon  characterized  by  an  increasing  amount 
of  free  ferrite. 

In  case  hardening  operations  the  penetration  of  the  carbon  may  be  very  slight 
indeed,  not  exceeding  0.5  mm.,  while  it  may  measure  as  much  as  5  mm.     In  the 


'ft 


Fig.  296.  —  Steel.    Case  hardened.    Magnified  100  diameters.    (G.  A.  Rein- 
hardt  in  the  author's  laboratory.) 


majority  of  instances  the  penetration  does  not  exceed  2  mm.  This  depth  of  pene- 
tration or  thickness  of  case  must  be  regulated  according  to  requirements.  It  will 
depend  upon  temperature,  time,  composition  of  steel,  and  kind  of  carburizing  mate- 
rial. Lake  mentions  0.87  mm.  per  hour  as  an  average  speed  of  penetration.  As 
already  stated,  it  is  not  generally  advisable  to  produce  a  case  containing  more  than 
some  0.90  per  cent  carbon. 

The  production  of  a  deep  case,  while  at  the  same  time  keeping  the  carbon  content 
of  the  outside  of  the  case  below  one  per  cent,  may  be  brought  about  by  a  rather  long 
treatment  at  a  relatively  low  temperature,  namely,  some  850  deg.  Some  results  ob- 
tained by  Guillet  in  regard  to  the  influence  of  temperature  and  of  time  on  the  depth 
of  penetration  are  shown  graphically  in  Figure  297  as  plotted  by  Bauer.  The  carbu- 
rizing material  used  was  not  stated.  The  full  line  represents  relative  penetrations  at 
1000  deg.  after  different  lengths  of  time,  namely,  one,  two,  four,  and  six  hours,  while 


318     CHAPTER  XIX  — THE  CEMENTATION  AND  CASE  HARDENING  OF  STEEL 

the  broken  line  represents  the  depths  of  penetration  resulting  from  heating  for  eight 
hours  at  different  temperatures. 

It  will  be  obvious  that  the  process  of  case  hardening  can  be  controlled  by  the 
microscopical  examination  of  test  pieces  much  more  readily  and  accurately  than  by 
chemical  analysis. 

Carburizing  Materials.  —  A  great  variety  of  carbonaceous  materials  is  used  for 
introducing  carbon  in  iron  and  steel  in  the  solid  state.  These  substances  may  be 
solid,  liquid,  or  gaseous.  Solid  materials  are  used  more  extensively  than  liquid  or 
gaseous  ones,  the  most  important  being  charcoal  (both  wood  and  bone),  charred 
leather,  crushed  bone,  horn,  mixtures  of  barium  carbonate  (40  per  cent)  and  char- 
coal (60  per  cent)  or  of  salt  (10  per  cent)  and  charcoal  (90  per  cent),  both  recom- 
mended by  Guillet,  and  for  quick  but  very  superficial  hardening,  powdered  potassium 
cyanide  and  potassium  ferro-cyanide  or  mixtures  of  potassium  ferro-cyanide  and 
potassium  bichromate.  A  molten  bath  of  potassium  cyanide  heated  to  850  deg.  and 
in  which  the  steel  articles  are  immersed  produces  quickly  superficial  but  hard  and 


900      4 


800     2 


700 


^/ 

^ 

x 

^^-; 

<  —       ' 

,-->*"' 

.** 

/ 

s 
y 

/ 
/ 

7 

IO  15  20  25 

PeneXraXian,  in-  m/m,. 


Fig.  297.  —  Temperature  and  time-penetration  curve.    (From  Brearley': 
"  The  Heat  Treatment  of  Tool  Steel.") 


even  cases.  The  poisonous  character  of  the  escaping  gases,  however,  is  a  serious  ob- 
jection to  the  use  of  this  method.  The  carburizing  of  iron  may  also  be  performed  at 
the  proper  temperature  by  means  of  gases  such  as  illuminating  or  other  coal  or  oil 
gases  rich  in  carbon  monoxide  and  in  hydrocarbons.  At  the  Krupp  Avorks  in  Ger- 
many gases  are  used  for  carburizing  the  faces  of  armor  plates.  Abbott  states  that 
out  of  100,000  tons  of  carburizing  material  used  in  the  United  States  in  1911,  85  per 
cent  was  granulated  bone. 

The  relative  merits  of  wood  charcoal,  charred  leather,  and  a  mixture  of  barium 
carbonate  and  of  wood  charcoal  for  carburizing  are  shown  graphically  in  Figure  298, 
in  which  are  plotted  some  results  obtained  by  Shaw-Scott.  While  wood  charcoal 
causes  a  slow  carburization  it  is  the  best  material  and  the  one  invariably  employed 
for  the  production  of  very  deep  cases  as,  for  instance,  in  making  blister  steel. 

Giolitti  surrounds  the  pieces  to  be  case  hardened  with  charcoal  and  then  passes 
through  the  annealing  box  carbon  monoxide  or  carbon  dioxide  gas,  the  latter  upon 
coming  in  contact  with  red  hot  charcoal  being  converted  into  carbon  monoxide 


CHAPTER  XIX  — THE  CEMENTATION  AND  CASE  HARDENING  OF  STEEL      319 

(C02  +  C  =  2CO)  which  gas  as  later  explained  is  the  most  active  agent  in  car- 
burizing.  In  Giolitti's  opinion,  however,  CO  when  used  alone  generally  fails  to  in- 
troduce enough  carbon,  but  becomes  much  more  effective  when  mixed  with  hydro- 
carbons or  better  still  with  finely  divided  solid  carbon.  Under  these  conditions  the 
maximum  concentration  does  not  exceed  0.90,  per  cent  carbon  even  after  8  hours  at 
a  constant  temperature  between  900  and  1100  deg.  C.  If  the  temperature  be  per- 
mitted to  fluctuate,  however,  between  1000  and  1100  deg.,  the  outer  layers  take  up 
as  much  as  1.10  per  cent  carbon  after  an  annealing  of  3  hours'  duration. 

Many  so-called  secret  mixtures  are  offered  for  sale  as  case  hardening  substances 
for  which  extraordinary  virtues  are  claimed,  the  usual  statement  being  that  by  their 
use  steel  of  ordinary  or  inferior  quality  may  be  converted  into  high  grade  metal  com- 


4.          ~6  a  10  12 

Time  of  Heating,  (hours) 

Fig.  298.  —  Time-penetration  curve.    (From  Brearley's  "The 
Heat  Treatment  of  Tool  Steel.") 

parable  to  the  best  crucible  tool  steel.  On  investigation  they  are  generally  found  to 
be  chiefly  mixtures  of  carbonaceous  and  cyanogen  compounds  possessing  the  well- 
known  carburizing  properties  of  those  substances. 

Case  Hardening  by  Gas  under  Pressure.  —  Dr.  F.  C.  Langenberg  has  con- 
ducted in  the  author's  laboratory  an  extensive  series  of  experiments  dealing  with 
the  case  hardening  of  American  ingot  iron  with  various  gases  under  varying  con- 
ditions of  temperature,  time,  flow  of  gas,  and  pressure.  Some  of  the  most  conclu- 
sive results  obtained  will  be  recorded. 

Illuminating  gas  of  the  following  composition: 


Hydrogen  40.10  per  cent 

Methane  26.10    "       " 

Nitrogen  14.04    "      " 

Carbon  Dioxide  1.64    "       " 


Oxygen  0.58  per  cent 

Carbon  Monoxide       11.38    "       " 
Heavy  Hydrocarbons  4.95 


was  passed  through  a  tube  furnace  electrically  heated  and  containing  near  the  in- 
let end  a  plug  of  finely  divided  charcoal  and,  immediately  following,  small  cylin- 


320     CHAPTER  XIX  —  THE  CEMENTATION  AND  CASE  HARDENING  OF  STEEL 

ders  of  American  ingot  iron  to  be  case  hardened.  The  passing  of  the  illuminating 
gas  through  the  highly  heated  charcoal  converted  the  C02  originally  present  into 
CO  (CO-2  +  C  =  2CO)  and  may  also  have  produced  some  changes  in  the  hydro- 
carbons present.  It  is  not  probable  that  the  hydrogen  content  was  greatly,  if  at 
all,  affected.  Maintaining  the  flow  of  gas  constant,  at  5  liters  per  hour,  the  tem- 
perature likewise  constant  at  990  deg.  C.  for  3  hours,  but  varying  the  pressure 
from  0  to  115  Ibs.  per  sq.  in.,  a  gradual  increase  in  the  carbon  absorbed  was  ob- 
served as  indicated  both  by  increase  of  weight  and  depth  of  case.  This  is  clearly 
shown  by  the  curves  of  Figure  299.  It  will  be  noted  that  the  increase  is  greatest 


15 

14 

13 
12 
II 
10 


Increase  of 
We  lent 
_or° 
Depth 
in  mm. 


6 
5 
4 
3 
2 
I 


F'ressure-Carbur/z&tion  Cur  ye 
Temperature    9/0" C. 

oj  C<5L  se  Curve 


Jncrea.se  of  We/gM  Cur-ye. 


0       10     20    30    40     50    60     70    80    90     100    110    120    130    I4O    150   160 
Pressi/«e-/6syber  sc/.in.. 

Fig.  299.  —  Case  hardening  by  gas.    Pressure-penetration  curve.     (F.  C.  Langenberg  in  the 

author's  laboratory.) 


as  the  pressure  rises  from  0  to  some  40  Ibs.  per  sq.  in.,  both  curves  for  higher 
pressure  showing  a  tendency  towards  an  horizontal  deflection.  This  occurrence 
was  found  to  be  still  more  marked  when  the  case-hardening  operation  was  con- 
ducted at  lower  temperatures.  In  every  instance,  however,  the  bend  in  the  curves 
occurred  in  the  vicinity  of  40  Ibs.  pressure.  The  photomicrographs  (Fig.  300 
to  302)  clearly  reveal  the  increasing  depth  of  case  resulting  from  increasing  gas 
pressure,  other  conditions  remaining  unchanged.  It  should  be  observed  that  it  is 
the  hyper-eutectoid  portion  of  the  case  which  is  chiefly  affected,  increasing  in  thick- 
ness with  the  pressure,  while  the  width  of  the  eutectoid  band  remains  practically 
the  same. 


CHAPTER  XIX  — THE  CEMENTATION  AND  CASE  HARDENING  OF  STEEL      321 


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322     CHAPTER  XIX  — THE  CEMENTATION  AND  CASE  HARDENING  OF  STEEL 

Maintaining  a  constant  atmospheric  pressure,  constant  flow  of  gas  (5  liters  per 
hour),  constant  time  (3  hours),  but  varying  the  temperature  from  600  to  1000  deg. 
C.,  the  curves  of  Figure  303  were  obtained.  They  indicate  the  increasing  amount  of 
carbon  taken  up  as  estimated  both  by  increasing  weight  and  increasing  depth  of  case. 
The  following  inferences  appear  justified:  (l)  at  700  deg.  C.  or  at  any  lower  tem- 
perature there  was  no  absorption  of  carbon  whatever,  from  which  it  may  be  con- 
cluded that  a  iron,  under  the  prevailing  experimental  conditions,  cannot  dissolve 
carbon,  (2)  at  800  deg.  C.  there  was  a  noticeable  carbon  absorption,  hence  the 


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Fig.  303.  —  Case  hardening  by  gas.    Temperature  —  penetration  curves.     (F.  C.  Langenberg  in 

the  author's  laboratory.) 


power  to  dissolve  carbon  is  acquired  between  700  and  800  deg.  C.,  that  is  during 
the  range  of  temperature  where  ft  iron  also  is  formed  and  it  may  be  reasonably 
believed  that  the  two  occurrences  are  closely  linked,  (3)  at  900  deg.  there  is  a 
break  in  both  curves  indicative  of  a  slower  rate  of  increase,  and  this  appears  to  be 
closely  related  to  the  formation  of  j  iron. 

Figure  304  shows  the  structure  of  American  ingot  iron  after  the  case-hardening 
operation  just  outlined  conducted  at  720  deg.  C.,  while  when  performed  at  810 
deg.  C.  the  structure  of  the  same  metal  was  as  represented  in  Figure  305.  There 
is  no  indication  of  the  former  having  absorbed  any  carbon  while  the  carburizing 
of  the  latter  is  very  appreciable. 

Mechanism  of  Cementation.  —  It  was  held  for  many  years  that  in  the  cementa- 
tion of  iron  solid  carbon  passed  bodily  from  the  packing  material  into  the  metal,  fol- 


CHAPTER  XIX  —  THK  CEMENTATION  AND  CASE  HARDENING  OF  STEEL     323 


'"    HP 

. 

' 

<     '  t>":rv' 

:   ••;* 


-•'     .  •"*."         i 

•  • 


Fig.  304.  —  American  ingot  iron  case  hardened  by  gas  for  3 
hours  at  720  deg.  C.  Magnified  100  diameters.  (F.  C.  Lan- 
genberg  in  the  author's  laboratory.) 


• 

*•  -         A 


.  ~~  •       . 


Fig.  305.  —  American  ingot  iron  case  hardened  by  gas  for  3 
hours  at  810  deg.  C.  Magnified  100  diameters.  (F.  C.  Lan- 
genberg  in  the  author's  laboratory.) 


324     CHAPTER  XIX  — THE  CEMENTATION  AND  CASE  HARDENING  OF  STEEL 

lowed  by  a  slow  migration  towards  the  center.  Recent  investigations,  however,  have 
made  it  evident  that  the  transfer  of  the  carbon  from  the  packing  material  to  the 
metal  is  accomplished  chiefly,  if  not  altogether,  by  means  of  some  gases  liberated  or 
formed  during  the  annealing  treatment.  It  has  been  shown  quite  conclusively,  for 
instance,  that  if  a  piece  of  steel  surrounded  by  pure  carbon  be  heated  in  vacuum, 
thus  precluding  the  formation  of  gases,  it  will  not  take  up  carbon,  although  one  ob- 
server has  noted  that  if  decided  pressure  be  applied  some  carbon  will  pass  into  the 
iron  even  in  the  absence  of  gases.  Whether  this  be  so  or  not  it  is  apparently  certain 
that  the  carbon  must  first  be  volatilized  before  becoming  very  active  as  a  carburiz- 
ing  agent  in  the  cementation  and  case  hardening  treatments. 

Carbon  monoxide  (CO)  and  volatilized  cyanogen  (CN)  compounds  are  the  gases 
which  seem  most  effective.  The  carbon  monoxide  is  derived  from  a  partial  combus- 
tion of  the  carbon  of  the  cementing  material  by  atmospheric  oxygen  while  the  cyan- 
ogen results  from  a  combination  of  that  carbon  with  atmospheric  nitrogen  or  from 
the  decomposition  of  cyanide  compounds  such,  for  instance,  as  potassium  cyanide 
and  ferro-cyanide.  It  may  be  assumed  that  the  carbon  monoxide  once  formed  gives 
up  its  carbon  to  the  iron  according  to  the  reaction, 

2CO  +  3Fe  =  Fe3C  +  CO2, 

the  resulting  Fe3C  or  cementite  being  dissolved  by  the  austenite  very  much  as  salt  is 
dissolved  in  water  and  the  C02  being  again  reduced  to  CO  on  coming  in  contact  with 
fresh  carbon  (C02  +  C  =  2CO).  The  marked  activity  of  cyanogen  compounds 
compared  to  the  slower  action  of  charcoal  have  led  some  to  believe  that  cyanogen 
gases  are  especially  effective  in  carburizing  iron.  It  should  be  noted,  however,  that 
while  cyanide  compounds  produce  a  much  quicker  carburization  they  soon  lose  their 
carburizing  power  so  that  when  deep  cases  are  needed,  as  in  the  manufacture  of 
blister  bars,  charcoal,  acting  chiefly  through  the  production  of  carbon  monoxide,  i.s 
preferable. 

Cooling  from  Carburizing  Temperature.  —  It  is  generally  desired  that  articles 
subjected  to  the  case  hardening  treatment  should  have  a  very  hard  surface.  To  pro- 
duce this  hardness  the  case  hardened  articles  should  be  quenched  from  above  their 
critical  range.  The  prolonged  heating  at  a  very  high  temperature  to  which  these 
articles  have  been  exposed,  however,  has  developed  a  coarseness  of  structure  both  in 
the  core  and  in  the  case  which  would  be  retained  if  they  were,  as  they  sometimes  are, 
quenched  from  the  carburizing  temperature  or  after  cooling  to  a  somewhat  lower 
temperature.  It  is  obvious  that  in  order  to  impart  a  fine  structure  both  to  the  core 
and  to  the  case  the  articles  should  be  cooled  and  then  subjected  to  suitable  heat 
treatments. 

Heat  Treatment  of  Case  Hardened  Articles.  —  In  order  to  refine  the  structure  of 
the  core  which  has  been  coarsened  by  a  long  exposure  to  a  high  temperature  the 
metal  should  be  reheated  slightly  above  the  critical  range  of  that  core  and  since  its 
carbon  content  seldom  exceeds  0.15  per  cent  carbon  a  temperature  of  at  least  900 
deg.  C.  should  be  used.  Guillet  recommends  1000  to  1025  deg.  The  finer  structure 
thus  imparted  to  the  core  will  then  be  retained  most  effectively  by  quenching  the 
metal  in  water  or  oil.  By  such  treatment,  however,  the  case,  although  hardened,  is 
still  relatively  coarse  since  its  quenching  was  effected  at  a  temperature  considerably 
exceeding  its  critical  range.  In  order  to  refine  it  while  leaving  the  structure  of  the 
core  undisturbed  the  article  should  now  be  reheated  slightly  above  the  critical  range 


CHAPTER  XIX  — THE  CEMENTATION  AND  CASE  HARDENING  OF  STEEL     325 

of  the  case,  that  is,  to  some  775  or  825  deg.  C.,  and  then  quenched  in  oil  or  water. 
By  this  double  treatment  we  have  hardened  the  case  while  conferring  to  it  as  well  as 
to  the  core  a  fine  structure. 

It  has  been  observed  that  the  low  carbon  steel  cores  of  case  hardened  articles  are 
frequently  coarser  than  the  same  steel  after  like  heat  treatment  conducted  in  the 
absence  of  carburizing  material.  This  is  generally  ascribed  to  the  action  of  hydrogen, 
most  carburizing  'agents  evolving  some  of  that  gas  at  the  annealing  temperature. 
This  increased  coarseness  of  the  cores  of  case  hardened  implements  is  an  additional 
reason  why  their  structure  should  be  refined  by  suitable  heat  treatments. 

Tempering  Case  Hardened  Steel.  —  The  properties  of  quenched  case  hardened 
steel  may  often  be  further  improved  by  tempering,  i.e.  by  reheating  to  some  200  to 
400  deg.  C.  in  order  to  toughen  the  case  and  remove  strains  while  losing  but  little 
hardness. 


CHAPTER  XX 

ALLOY  STEELS 

GENERAL  CONSIDERATIONS 

The  steels  so  far  considered  in  these  chapters  are  the  ordinary  steels  of  commerce, 
at  present  often  called  "carbon"  steels  to  distinguish  them  from  the  "special"  or 
"alloy"  steels  of  relatively  recent  origin  but  of  rapidly  growing  importance.  By 
special  steels  is  meant  those  steels  which  owe  their  properties  in  a  marked  degree  to 
the  presence  of  one  or  more  special  elements  whereas  the  properties  of  carbon  steels 
depend  chiefly,  if  not  exclusively,  for  like  treatment,  upon  the  proportion  of  carbon 
present.  Alloy  steels  containing  but  one  special  element  are  commonly  called  "ter- 
nary" steels,  being  considered  to  be  made  up  of  three  constituents,  namely  iron,  car- 
bon, and  the  special  element,  while  steels  containing  two  special  elements  are  called 
" quarternary "  steels  because  of  the  presence  of  four  constituents:  iron,  carbon,  and 
the  two  special  elements.  These  two  classes  of  special  steels  will  be  considered 
separately. 

Ternary  Steels.  —  We  are  indebted  to  Guillet  for  a  brilliantly  conceived  and 
vigorously  developed  theory  of  the  ternary  steels.  Too  rigorous  an  application  of  the 
theory,  however,  should  not  be  insisted  upon  for  there  are  some  facts  not  yet  satis- 
factorily explained  by  it.  Its  use,  nevertheless,  will  be  found  an  invaluable  guide 
in  directing  researches  dealing  with  the  manufacture  and  the  application  of  these 
steels. 

Guillet's  theory  of  the  structure  and  properties  of  ternary  steels  may  be  briefly 
formulated  by  a  few  propositions.  It  is  also  represented  graphically  in  Figure  306. 

(1)  On  the  introduction  of  a  special  element  in  carbon  steel  the  latter  remains  at 
first  pear li tic,  but  as  the  proportion  of  the  special  element  increases,  the  carbon  re- 
maining constant,  it  becomes  first  martensitic  and  then  austenitic  (polyhedral),  as 
shown  graphically  in  Figure  306,  and  sometimes  cementitic  (carbide  steel)1  as  later 
explained. 

(2)  By  increasing  the  amount  of  carbon  present  in  a  special  steel,  the  proportion 
of  the  special  element  being  kept  constant,  it  is  generally  converted  from  a  pearlitic 
into  a  martensitic  condition  or,  if  already  martensitic,  into  an  austenitic  condition. 

(3)  The  greater  the  amount  of  carbon  the  smaller  the  proportion  of  the  special 
element  needed  to  cause  a  structural  transformation,  as  for  instance  pearlite  into  mar- 
tensite  or  martensite  into  austenite.    This  is  indicated  in  Figure  306. 

1  Guillet  uses  the  term  "polyhedral"  to  designate  an  austenitic  structure  and  "carbide"  steel  (acier 
a  carbure)  to  indicate  the  presence  of  cementite  (generally  in  special  steels  a  double  carbide  of  iron  and 
the  special  element) .  It  seems  to  the  author  that  the  terms  austenitic  and  cementitic  are  preferable 
because  they  suggest  unmistakably  the  nature  of  the  constituents.  Austenitic  steels  are  not  the  only 
ones  exhibiting  a  polyhedral  structure;  ferritic  (low  carbon)  steels  for  instance  are  also  polyhedral. 

326 


CHAPTER  XX  — ALLOY   STEELS  327 

(4)  The  greater  the  amount  of  the  special  element  the  smaller  the  proportion  of 
carbon  needed  to  cause  a  structural  transformation.    This  is  also  shown  in  Figure  306. 

(5)  No  very  sharp  lines  of  demarcation  are  observed  between  the  different  types 
of  structures  mentioned  in  the  preceding  propositions,  relatively  wide  ranges  of  com- 
position existing,  on  the  contrary,  in  which  the  steel  may  be  partly  pearlitic  and  partly 
martensitic  or  partly  martensitic  and  partly  austenitic,  etc.    These  transition  ranges 
are  indicated  by  shaded  areas  in  the  diagram  of  Figure  306.    Greater  refinement  in 
the  construction  of  this  diagram  would  undoubtedly  lead  to  the  introduction  of  a 
troostitic  zone  between  the  pearlite  and  martensite  areas  and  possibly  also  of  a  sor- 
bitic  zone  between  pearlite  and  troostite. 

To  sum  up,  constituents  may  be  formed  during  the  slow  cooling  of  many  alloy 
steels  which  in  carbon  steels  can  only  be  produced  by  very  rapid  cooling  through  the 


.2  <*  .6  .5  i.o  /.2  14  is 

Percent     carbon. 

* 

Fig.  306.  —  Constitutional  diagram  of  alloy  steels. 

critical  range.  Carbon  steels,  moreover,  even  after  very  rapid  cooling  cannot  be  re- 
tained wholly  in  an  austenitic  condition  while  several  special  steels  remain  austenitic 
after  slow  cooling.  It  is  evident  from  the  above  and  from  the  diagram  that  in  order 
to  produce  a  certain  structure,  (1)  the  proportion  of  carbon  may  be  kept  constant 
while  the  proportion  of  the  special  element  is  increased  until  the  desired  structure  is 
obtained,  or  (2)  the  proportion  of  the  special  element  may  be  kept  constant  and  the 
proportion  of  carbon  increased,  or  (3)  both  the  proportion  of  carbon  and  of  the  special 
element  may  be  increased  when  the  desired  structure  will  be  obtained  more  quickly. 
The  usefulness  of  Quillet's  diagram  is  obvious.  Should  we  desire,  for  instance,  to 
know  the  kind  of  structure,  and  therefore  the  physical  properties,  of  a  steel  contain- 
ing 0.60  per  cent  carbon  and  8  per  cent  of  the  special  element,  the  diagram  shows 
that  such  composition  falls  within  the  martensitic  range.  Likewise  a  steel  contain- 


328  CHAPTER  XX  — ALLOY   STEELS 

ing  one  per  cent  carbon  and  15  per  cent  of  the  special  element  would  be  austenitic 
according  to  the  diagram.  Or  one  may  wish  to  know  what  proportion  of  the  special 
element  should  be  added  to  a  carbon  steel  containing,  say,  0.5  per  cent  carbon,  to 
make  it  martensitic;  the  diagram  shows  that  7  per  cent  will  be  needed.  Again,  hav- 
ing an  austenitic  steel  containing  10  per  cent  of  the  special  element  it  may  be  desired 
to  know  the  minimum  amount  of  carbon  that  may  be  present  without  causing  the 
steel  to  become  martensitic;  the  diagram  shows  0.80  per  cent  of  carbon  to  be  the 
smallest  proportion  of  carbon  permissible. 

The  construction  of  such  diagrams  requires  the  preparation  of  a  number  of  alloys 
varying  in  their  contents  of  carbon  and  of  the  special  element,  their  microscopical 
examination  and  the  plotting  of  their  structure. 

It  is  quite  essential  to  know  the  rate  of  cooling  adopted  in  the  construction  of  the 
diagram,  i.e.  whether  the  samples  were  cooled  in  air  or  more  slowly  in  the  furnace, 
for  it  is  evident  that  their  structure  may  be  deeply  affected  by  thus  varying  the  speed 
at  which  they  cool.  Some  special  steels,  for  instance,  may  be  pearlitic  when  cooled 
very  slowly  in  the  furnace,  martensitic  when  cooled  in  air,  and  austenitic  after  water 
quenching. 

Influence  of  the  Special  Element  upon  the  Location  of  the  Critical  Range.  —  The 
production  of  martensitic  and  austenitic  structures  on  slow  cooling  is  due  to  the  fact 
that  the  special  element  lowers  the  position  of  the  critical  point  to  a  temperature  so 
low  (1)  as  to  permit  only  a  partial  transformation,  namely,  of  austenite  into  marten- 
site,  the  steel  being  too  rigid  to  allow  a  more  complete  transformation,  or  (2)  as  to 
prevent  even  a  slight  transformation,  the  steel  in  that  case  remaining  austenitic. 
This  influence  of  the  special  element  in  lowering  the  position  of  the  critical  range  is 
depicted  in  Figure  307  in  which  it  is  assumed  that  the  proportion  of  carbon  remains 
constant.  It  has  been  further  arbitrarily  assumed  in  this  diagram  that  the  critical 
point  was  progressively  and  uniformly  lowered  from  700  deg.  C.  to  0  deg.,  as  the  pro- 
portion of  the  special  element  increased  from  0  to  6  per  cent.  From  many  observa- 
tions it  appears  (1)  that  as  long  as  the  critical  point  remains  above  300  deg.  C.  the 
steel  becomes  pearlitic  on  slow  cooling,  (2)  that  when  the  critical  point  is  lowered 
below  300  deg.  it  becomes  martensitic,  the  rigidity  of  the  metal  preventing  further 
transformation,  and  (3)  that  when  the  critical  point  is  lowered  to  atmospheric  tem- 
perature or  below  it  the  metal  remains  untransformed,  that  is  austenitic.  These 
inferences  are  offered  here  because  of  their  apparent  usefulness  and  suggestiveness, 
but  the  author  realizes  that  the  lines  indicating  the  relation  between  the  position  of 
the  critical  points  and  the  corresponding  structures  cannot  be  sharply  drawn,  for 
they  are  likely  to  shift  according  to  the  nature  of  the  special  element,  the  rate  of 
cooling,  etc.  Again,  troostitic  and  possibly  also  sorbitic  steel  are  likely  to  form  be- 
tween pearlite  and  martensite,  that  is,  whenever  the  critical  point  is  lowered,  say 
below  400  or  possibly  below  500  deg. 

To  make  the  meaning  of  the  diagram  of  Figure  307  clear  let  us  consider  three 
steels:  I,  II,  and  III,  all  containing  one  per  cent  of  carbon,  but  respectively  1,  4.50, 
and  7  per  cent  of  the  special  element.  As  steel  I  cools  it  undergoes  its  transformation 
at  about  600  deg.  At  that  temperature  the  metal  is  so  plastic  that  the  transformation 
of  austenite  into  pearlite  readily  takes  place;  the  steel  becomes  pearlitic.  The  critical 
point  of  steel  II  is  slightly  below  200  deg.  At  this  temperature  the  transformation 
of  austenite  into  martensite  will  take  place,  but  the  metal  is  now  too  rigid  to  permit 
further  transformations ;  the  steel  remains  martensitic.  Steels  which  remain  marten- 


CHAPTER  XX  — ALLOY   STEELS 


329 


sitic  after  slow  (air)  cooling  are  said  to  be  "self-hardening."  In  the  case  of  steel 
III,  since  its  critical  point  is  lowered  below  atmospheric  temperature  it  necessarily 
remains  austenitic.  Since  austenitic  special  steels  have  their  points  of  transformation 
situated  below  atmospheric  temperature,  it  should  be  possible  through  cooling  to  a 
sufficiently  low  temperature,  as  for  instance  by  immersion  in  liquid  air,  to  cause  at 
least  their  partial  transformation,  that  is,  they  should  become  martensitic  after  such 
treatment  and  this  indeed  is  precisely  what  happens  in  some  cases.  The  transforma- 


800 


TOO 


F 


I  ^  3  4  5  6N 

°/a<Special    element. 
Carbon    I  % 

Fig.  307.  —  Influence  of  special  element  on  the  position  of  the  critical  point. 


8 


tion  of  austenite  into  martensite  takes  place  with  increased  volume  and  the  steel 
from  non-magnetic  becomes  magnetic. 

In  the  presence  of  a  special  element  lowering  the  critical  points  the  influence  of 
carbon  is  cumulative,  i.e.  the  greater  the  proportion  of  carbon  the  more  marked  the 
action  of  the  special  element.  An  attempt  has  been  made  in  Figure  308  to  show  this 
graphically.  The  diagram  indicates  the  position  of  the  critical  point  corresponding 
to  any  combination  of  carbon  content  between  0.25  and  1.50  and  of  the  special  ele- 


330 


CHAPTER  XX  — ALLOY  STEELS 


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o. 


.1 


1 


y 

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CHAPTER  XX  — ALLOY   STEELS  331 

ment  between  0  and  12  per  cent.  If  the  point  is  above  300  deg.  we  may  assume  that 
the  steel  is  pearlitic,  if  below  300  that  it  is  martensitic,  if  at  or  below  atmospheric 
temperature  that  it  is  austenitic.  In  this  illustration  arbitrary  values  have  been 
given  to  the  combined  influence  of  various  proportions  of  carbon  and  of  the  special 
element  upon  the  position  of  the  critical  points.  The  diagram  shows,  for  instance, 
that  while  with  0.50  per  cent  carbon  10  per  cent  of  the  special  element  are  required  to 
lower  the  critical  points  to  0  deg.  C.,  if  the  steel  contains  1.25  per  cent  carbon,  4  per 
cent  of  the  special  element  suffice.  The  production  of  pearlitic,  martensitic,  and 
austenitic  structures  according  to  the  position  of  the  critical  point  has  also  been  in- 
dicated. A  diagram  of  this  kind  may  be  even  more  useful  than  Guillet's  for,  while 
giving  the  same  kind  of  information  as  his,  it  shows  in  addition  (1)  the  relation  be- 
tween the  composition  of  the  special  steel  and  the  position  of  the  critical  point  and 
(2)  the  influence  of  the  position  of  the  critical  point  upon  the  structure.  Its  construc- 
tion calls  for  many  determinations  of  the  position  of  the  critical  point  in  steels  of  vary- 
ing composition  and  for  the  microscopical  examination  of  the  corresponding  structures. 
It  is  of  course  quite  likely  that  as  experimentally  constructed  it  would  consist  of  more 
or  less  smooth  curves  rather  than  of  straight  lines. 

It  will  soon  be  shown  that  the  influence  of  special  elements  in  lowering  the  critical 
points  varies  greatly,  from  which  it  follows  that  some  elements  cause  the  production 
of  martensitic  and  austenitic  steels  much  more  readily  than  others.  Indeed  some 
elements  never  cause  a  sufficient  depression  of  the  critical  points  to  yield  austenitic 
or  even  martensitic  steels.  A  few  elements  even  actually  raise  the  location  of  the 
critical  points  in  which  case,  of  course,  the  steel  always  becomes  pearlitic  on  slow 
cooling,  regardless  of  its  composition.1 

From  the  foregoing  considerations  it  appears  that  according  to  their  structural 
composition  special  steels  may  be  divided  into  at  least  four  classes,  (1)  pearlitic  steels, 
(2)  martensitic  steels,  (3)  austenitic  (polyhedral)  steels,  and  (4)  cementitic  (Guillet's 
carbide)  steels.  These  should  now  be  farther  considered. 

Pearlitic  Steels.  —  The  pearlitic  steels,  as  we  have  seen,  are  those  which  generally 
contain  but  a  relatively  small  amount  of  the  special  element,  although  in  case  of  steel 
very  low  in  carbon  the  proportion  of  the  special  element  may  be  quite  large.  In 
these  steels  the  special  element  may  (1)  be  dissolved  in  the  ferrite  forming  with  it  a 
solid  solution,  (2)  be  combined  with  carbon  in  cementite  as  a  double  carbide  of  iron 
and  the  special  element,  or  (3)  be  partly  dissolved  in  ferrite  and  partly  combined 
with  carbon. 

According  to  Quillet  nickel  and  silicon,  for  instance,  are  entirely  dissolved  in 
ferrite,  while  manganese,  chromium,  tungsten,  vanadium,  and  molybdenum  are 
partly  held  in  solution  by  ferrite  and  partly  present  in  cementite  as  double  carbides. 
Such  terms  as  nickel-ferrite,  silico-ferrite,  mangano-ferrite,  etc.,  have  been  suggested 
to  designate  ferrite  holding  in  solution  large  proportions  of  nickel,  silicon,  manga- 
nese, etc.,  respectively. 

The  structure  of  pearlitic  alloy  steels  is  generally  quite  similar  to  that  of  pearlitic 
carbon  steels,  although  the  pearlite  particles  of  special  steels  frequently  are  more 
angular  than  those  of  carbon  steel,  often  exhibiting  many  straight  sides  and  sharp 
corners,  whereas  the  pearlite  particles  of  carbon  steel  are  more  rounded.  The  lami- 

1  It  has  been  claimed  by  some  that  in  certain  alloy  steels  exhibiting  the  two  points  Aj  and  At 
while  A2  always  remains  at  the  same  temperature,  Ai  may  be  raised  above  A2  through  suitable  reg- 
ulation of  composition  and  heat  treatment. 


332  CHAPTER  XX  — ALLOY  STEELS 

nation  of  pearlite  is  also  often  more  minute  in  special  steels  while  for  same  carbon 
content  it  often  appears  to  occupy  a  larger  bulk.  From  their  similarity  of  struc- 
ture, it  might  reasonably  be  inferred  that  pearlitic  special  steels  should  not  differ 
much  in  physical  properties  from  ordinary  carbon  steels.  As  a  matter  of  fact,  how- 
ever, pearlitic  special  steels  are  often  greatly  superior  to  carbon  steels  generally  be- 
cause they  possess  in  a  much  greater  degree  that  desirable  combination  of  properties, 
strength,  or  rather  high  elastic  limit,  and  ductility.  They  are  also  frequently  harder 
for  like  ductility,  and  therefore  better  adapted  to  resist  wear.  Finally  their  ability 
to  resist  shocks  is  often  markedly  superior  to  that  of  carbon  steels.  Their  uses  make 
for  greater  efficiency  and  their  greater  strength  permits  an  often  welcome  reduction 
in  bulk  and  weight  of  certain  parts  of  machinery.  This  greater  strength  and  stiff- 
ness of  special  pearlitic  steels  may  be  due  to  the  special  element  dissolving,  in  part  at 
least,  in  the  ferrite  thereby  increasing  its  strength,  elastic  limit,  and  hardness,  a 
stronger  and  harder  ferrite  resulting  in  turn  in  a  stronger  and  harder  pearlite.  The 
superior  physical  qualities  of  these  steels  may  also  be  due,  at  least  partly,  to  a  finer, 
closer  ferrite-cementite  aggregate. 

The  critical  points  of  special  pearlitic  steels  generally  occur  at  temperatures  some- 
what lower  than  those  at  which  the  critical  points  of  carbon  steel  are  located,  this 
being  in  accord  with  the  usual  influence  of  special  elements  upon  these  points  as 
already  explained. 

Martensitic  Steels.  —  For  the  same  carbon  content  martensitic  steel  contains 
necessarily  more  of  the  special  element  than  pearlitic  steels,  while  for  a  given  propor- 
tion of  the  special  element  they  must  contain  more  carbon;  they  generally  contain 
both  more  carbon  and  more  of  the  special  element  than  pearlitic  steels.  As  already 
stated  the  influence  of  some  special  elements  in  lowering  the  critical  points  is  not 
sufficiently  pronounced  to  result  in  the  formation  of  martensite  on  slow  cooling. 
Indeed  some  elements  raise  the  position  of  the  critical  points  in  which  case  pearlitic 
steel  must  necessarily  always  be  formed  on  slow  cooling. 

The  properties  of  martensitic  special  steels  are  not  unlike  the  properties  of  marten- 
sitic carbon  steels,  that  is  of  hardened  carbon  steel.  These  steels  are  hard  and  brittle 
and  unforgeable  in  the  cold.  Their  uses  are  very  limited,  chiefly  because  of  their 
brittleness  and  of  the  difficulty  of  machining  them.  While  resembling  hardened  car- 
bon steels  they  are  quite  stable  above  atmospheric  temperature,  being  little  affected 
by  tempering,  i.e.  by  reheating  to  200  or  300  deg.  C.  This  property  suggests  one  im- 
portant application  at  least  of  martensitic  special  steels  later  to  be  considered,  namely, 
their  use  for  the  manufacture  of  cutting  tools,  their  greater  stability  permitting  the 
tools  to  be  heated  to  a  higher  temperature,  i.e.  the  cutting  being  performed  at  greater 
speed  without  breaking  down  through  excessive  tempering. 

The  martensite  of  special  steels  probably  is,  like  the  martensite  of  carbon  steels, 
chiefly  a  solid  solution  in  beta  iron  of  the  carbide  Fe3C  or  more  often  of  a  double  car- 
bide of  iron  and  of  the  special  element,  the  magnetism  of  the  metal  being  due  to  the 
presence  of  some  alpha  iron. 

Austenitic  (Polyhedral)  Steels.  —  For  a  given  carbon  content  austenitic  steels 
necessarily  contain  more  of  the  special  element  than  martensitic  steels,  while  for  a 
given  proportion  of  the  special  element  they  are  necessarily  more  highly  carburized. 
Austenitic  steels  generally  contain  both  more  carbon  and  more  of  the  special  element 
than  martensitic  steels.  Their  properties  are  as  might  be  expected  similar  to  those  of 
austenitic  carbon  steels,  that  is,  of  high  carbon  steels  cooled  extremely  quickly  from 


CHAPTER  XX  — ALLOY   STEELS  333 

a  very  high  temperature.  Austenitic  steels  are  moderately  tenacious  but  very  duc- 
tile; they  have  a  low  elastic  limit  but  possess  a  remarkable  power  of  resisting  wear  by 
abrasion  as  well  as  rupture  by  shocks.  The  mineralogical  hardness,  however,  is 
generally  inferior  to  that  of  martensitic  steels. 

Unlike  quenched  austenitic  carbon  steel  these  special  steels  are  stable  at  all  tem- 
peratures below  their  point  of  solidification  and  are  not  therefore  greatly  affected  by 
heat  treatment  unless  of  a  protracted  nature.  They  should  be  free,  however,  from 
separated  carbides.  If  not  they  should  be  heated  to  a  high  temperature  so  as  to  cause 
the  solution  of  these  carbides,  and  then  quenched  to  prevent  theineparating  again. 

The  austenite  of  special  steels  undoubtedly  consists  of  a  solid  solution  in  gamma 
iron  of  carbon  and  of  the  special  element,  probably  of  a  double  carbide.  Because  of 
the  absence  of  alpha  iron  austenitic  steels  are  non-magnetic. 

Austenitic  special  steels  find  useful  application  for  parts  of  machinery  and  the  like 
subjected  to  very  severe  wear  by  abrasion  and  to  shocks.  Their  low  elastic  limit  and 
the  difficulty  of  machining  them  are  the  chief  reasons  preventing  their  wider  use. 

Cementitic  (Carbide)  Steels.  —  Some  special  elements  on  being  introduced  in 
increasing  proportions  fail  to  convert  the  metal  into  austenite,  free  particles  of  a 
double  carbide  of  iron  and  the  special  element  being  formed  instead  and  embedded  in 
a  martensitic,  troostitic,  sorbitic,  or  pearlitic  matrix.  Guillet  calls  these  "carbide" 
steels.  Such  elements  as  chromium,  tungsten,  molybdenum,  and  vanadium  when 
present  in  sufficient  quantity  produce  cement  it  ic  steels.  The  most  valuable  prop- 
erty of  these  steels  is  their  power,  when  the  carbides  are  embedded  or  rather  dissolved 
in  a  martensitic  or  austenitic  matrix,  of  retaining  their  hardness  when  heated  to  such 
temperature  as  would  readily  cause  the  softening  of  hardened  carbon  steel,  thus 
permitting  their  use  in  the  shape  of  tools  at  such  speed  as  to  cause  their  cutting  edges 
to  become  visibly  hot.  This  phenomenon  will  be  further  explained  when  describing 
self-hardening  and  high  speed  steels. 

Treatments  of  Special  Steels.  —  Special  steels  are  subjected  to  the  same  treat- 
ments as  carbon  steels,  i.e.  to  hot  and  cold  working,  annealing,  hardening,  and  tem- 
pering, and  to  case  hardening.  Since  the  special  elements,  however,  often  have  a 
marked  influence  on  the  position  of  the  critical  points  it  is  obvious  that  the  tempera- 
tures indicated  as  the  most  suitable  ones  for  the  annealing  and  hardening  of  carbon 
sleds  may  not  be  satisfactory  in  the  case  of  special  steels.  The  position  of  the  critical 
points  should  in  every  case  be  determined  and  the  heat  treatments  conducted  accord- 
ingly. Greater  care  is  also  frequently  needed  in  the  forging  of  special  steels,  many 
of  them  not  being  quite  as  malleable  as  carbon  steels.  Finally  some  of  the  special 
elements  promote  the  absorption  of  carbon  by  iron  below  its  solidification-point  while 
others  oppose  it  and  these  influences  must  be  considered  in  case  hardening  special 
steels.  The  treatment  of  special  steels  will  be  considered  further  in  the  next  chapter 
in  connection  with  the  description  of  some  of  the  most  important  commercial  types. 

Treatment  of  Pearlitic  Steels.  —  Pearlitic  special  steels  may,  like  carbon  steels, 
be  subjected  to  annealing,  hardening,  tempering,  and  case  hardening.  Their  critical 
points,  however,  being  generally  lower,  the  proper  temperatures  for  these  operations 
are  likewise  lower.  They  should  be  determined  for  each  steel.  According  to  Guillet, 
however,  the  steel  should  be  heated  quite  a  little  above  its  critical  range  because  in 
the  case  of  the  pearlite  of  special  steels  its  transformation  into  a  solid  solution  does 
not  take  place  as  readily.  The  steel  should  then  be  cooled  to  near  its  critical  range 
before  quenching.  Guillet  writes  that  pearlitic  special  steels  may  be  divided  into 


334  CHAPTER  XX  — ALLOY  STEELS 

(1)  those  that  are  not  very  sensitive  to  annealing,  namely  nickel  and  silicon  steels, 
and  (2)  those  that  are  very  sensitive  to  annealing,  namely,  manganese,  chrome,  van- 
adium, tungsten,  and  molybdenum  steels.  It  should  be  noted  that  in  the  first  group 
the  special  elements  are  supposed  to  be  entirely  dissolved  in  the  iron,  while  in  the 
second  group  they  are  partly  dissolved  and  partly  present  as  carbides.  The  case 
hardening  of  pearlitic  special  steels  may  result  in  the  production  of  martensitic  or 
even  austenitic  cases  without  the  necessity  of  rapid  cooling  from  above  the  critical 
range.  This  will  be  readily  understood  by  referring  to  Figure  306  where  it  will  be 
seen  that  by  keeping  the  proportion  of  the  special  element  constant  and  increasing 
the  carbon  the  steel  may  be  converted  from  a  pearlitic  to  a  martensitic  and  even  to 
an  austenitic  condition.  The  nearer  the  steel  to  the  boundary  between  the  pearlitic 
and  martensitic  zones  the  more  readily,  of  course,  will  it  become  martensitic  on  case 
hardening  because  the  smaller  the  amount  of  carbon  needed  to  produce  that  trans- 
formation. This  possibility  of  producing  steel  objects  with  a  pearlitic  soft  core  and 
a  hard  martensitic  shell  without  quenching  from  a  high  temperature  and  therefore 
without  exposing  the  objects  to  the  dangers  of  the  quenching  bath  does  not  seem  to 
have  received  the  attention  it  deserves  for  it  suggests  important  practical  applica- 
tions. And  likewise  the  production  of  a  soft  pearlitic  core  surrounded  by  a  hard 
martensitic  steel,  itself  surrounded  by  a  tough  austenitic  steel.  It  points  at  least  to 
the  manufacture  of  pearlitic  steel  objects  which  can  be  readily  machined,  etc.,  and 
as  a  last  treatment  made  austenitic  to  a  certain  depth,  being  in  this  way  greatly 
superior  to  the  austenitic  steels  at  present  used,  which  being  cast  in  an  austenitic 
condition  can  be  machined  only  with  very  great  difficulty. 

Treatment  of  Martensitic  Steels.  —  Unlike  martensitic  carbon  steels  martensitic 
special  steels  being  quite  stable  below  the  critical  range  of  the  metal  are  not  readily 
affected  by  tempering  treatments.  It  should  be  borne  in  mind,  however,  that  some 
special  steels  which  are  martensitic  after  air  cooling  may  become  pearlitic,  in  part  at 
least,  after  very  slow  cooling  in  the  furnace  and  austenitic  or  martensito-austenitic 
after  quenching  in  water.  By  selecting  a  special  steel  of  suitable  composition,  for  in- 
stance, and  allowing  it  to  cool  in  a  furnace  it  becomes  pearlitic  and  can  in  consequence 
be  machined;  after  machining  the  finished  object  may  be  made  martensitic  by  cool- 
ing in  air,  doing  away  with  the  necessity  of  the  quenching  bath  and  its  inherent  evils. 
It  is  evident  that  for  this  purpose  the  composition  of  the  steel  should  be  near  the 
boundary  line  between  the  pearlitic  and  martensitic  zones.  The  author  believes  that 
the  practical  possibilities  of  this  procedure  have  been  overlooked.  When  working 
near  the  pearlito-martensitic  boundary  line  the  formation  of  troostite  is  of  course 
always  likely. 

Treatment  of  Austenitic  Steels.  —  Austenitic  special  steels  are  stable  theoretic- 
ally at  least  at  all  temperatures  and  should  not,  therefore,  be  affected  by  heat  treat- 
ment of  any  kind.  Some  special  steels,  however,  may  require  air  cooling  to  be  truly 
austenitic,  in  which  case  very  slow  cooling  in  the  furnace  may  result  in  the  produc- 
tion of  some  martensite  or  troostite  and  even  of  some  pearlite,  accompanied  by  the 
reappearance  of  magnetism.  It  also  frequently  happens  that  during  the  relatively 
slow  cooling  of  austenitic  steels  some  free  cementite  may  be  formed,  consisting  gen- 
erally of  a  double  carbide  of  iron  and  the  special  element,  this  setting  free  of  cemen- 
tite being  generally  accompanied  by  a  decided  decrease  of  strength  and  ductility. 
In  order  to  cause  the  reabsorption  of  the  separated  carbide  heating  to  a  high  tem- 
perature (1000  deg.  C.  or  higher)  is  generally  required  followed  by  rapid  cooling  in 


CHAPTER  XX  — ALLOY   STEELS  335 

water  or  oil  so  as  to  prevent  its  separating  again  on  cooling.  This  treatment  is  some- 
times called  "water  toughening." 

Treatment  of  Cementitic  Steels.  —  Cementitic  steels  contain  many  particles  of 
cementite  or  double  carbide  embedded  in  a  matrix  which  may  be  austenitic,  mar- 
tensitic,  troostitic,  sorbitic,  or  pearlitic  according  to  the  rate  of  cooling.  It  is  often 
desirable  to  cause  the  disappearance  in  part  at  least  of  these  particles  while  produc- 
ing a  finely  martensitic  or  austenitic  structure,  and  for  this  purpose  heating  to  a  high 
temperature  (1000  deg.  C.  or  more)  followed  by  relatively  quick  cooling  is  necessary. 
Cooling  in  air  is  often  sufficiently  rapid  to  retain  the  carbide  in-solution,  as  for  in- 
stance in  the  case  of  the  high  speed  steels  soon  to  be  described. 

Quaternary  Steels.  —  Quaternary  steels  like  ternary  steels  may  be  pearlitic,  mar- 
tensitic, austenitic,  or  cementitic  as  well  as  sorbitic  and  troostitic.  If  the  two  special 
elements  are  present  in  small  quantities  the  steels  remain  pearlitic.  If  they  contain 
one  or  two  cementite  forming  elements  such  as  chromium,  tungsten,  molybdenum, 
etc.,  they  are  likely  to  be  cementitic,  that  is,  to  contain  many  particles  of  a  double  or 
triple  carbide.  These  should  generally  be  made  to  dissolve  in  the  matrix  by  heating 
to  a  high  temperature  followed  by  rapid  cooling  when  a  finely  martensitic  or  aus- 
tenitic structure,  quite  free  from  cementite,  may  be  produced  as  in  the  treatment  of 
high  speed  steel.  If  the  quaternary  steels  contain  considerable  proportions  of  two 
special  elements  capable  of  forming  solid  solutions  with  iron,  as  for  instance  nickel 
and  manganese,  they  are  frequently  martensitic  or  austenitic.  A  large  proportion  of 
an  element  which  is  partly  dissolved  in  ferrite  and  partly  present  in  cementite  as  a 
double  carbide,  manganese,  for  instance,  may  result  in  the  occurrence  of  cementite 
particles  embedded  in  an  austenitic  matrix. 


CHAPTER  XXI 


ALLOY  STEELS 

CONSTITUTION,   PROPERTIES,   TREATMENT,   AND  USES  OF  MOST 

IMPORTANT  TYPES 

The  present  chapter  is  devoted  to  a  brief  consideration  of  the  composition,  struc- 
ture, properties,  treatments,  and  uses  of  those  special  steels  which  have  been  found  to 
be  of  commercial  value,  namely,  nickel,  manganese,  tungsten,  chromium,  vanadium, 

JO. . 


o 


Austin  if  ic 


Marten  sit ic 


Pearlit'i  c 


O  OA  O.Q  1.2.  /.6 

%Carbon 

Fig.  309.  —  Nickel  steel.     Constitutional  diagram.     (Guillet.) 

silicon,  molybdenum,  chrome-nickel,  chrome-vanadium,  chrome-tungsten,  and  chro- 
memolybdenum  steels. 

Nickel  Steel.  —  Nickel  apparently  dissolves  in  iron  in  all  proportions.  The  con- 
stitutional diagram  of  nickel  steel  is  illustrated  in  Figure  309  after  Guillet.  In  view 
of  the  explanation  of  such  diagrams  given  in  the  preceding  chapter  it  will  be  readily 
understood.  It  shows  that  as  the  carbon  increases  from  0  to  1.60  per  cent  and  the 

336 


CHAPTER  XXI  — ALLOY   STEELS 


337 


nickel  from  0  to  30  per  cent  the  metal  which  at  first  remains  pearlitic  becomes  mar- 
tensitic,  and  finally  austenitic.  The  nickel  steels  of  greatest  commercial  importance 
seldom  containing  more  than  5  per  cent  nickel  and  one  per  cent  carbon  are  pearlitic. 
This  influence  of  nickel  in  preventing  partly  or  wholly  the  transformation  of  austenite 
into  pearlite  is  due  to  its  lowering  the  critical  point  of  the  steel  as  fully  explained  in 
the  last  chapter  and  as  illustrated  graphically  in  Figure  310  in  the  case  of  iron-nickel 
alloys  containing  small  proportions  of  carbon.  It  should  be  remembered  that  the 
presence  of  larger  quantities  of  carbon  would  intensify  the  influence  of  nickel,  i.e. 
would  cause  the  critical  points  to  be  further  depressed.  The  diagram  shows  that  as 
the  nickel  increases  from  0  to  some  25  per  cent  both  transformations,  AB  on  cooling 
and  A  'B'  on  heating,  are  depressed,  the  former,  however,  much  more  quickly  than 
the  latter,  resulting  in  a  rapidly  increasing  gap  between  the  two  transformations.  In 


A 
A 

700 


600 


too 


-100 


V 


II 


10         90         30         <>0         50         60         10         80         80         IOQ 


Fig.  310.  —  Influence  of  .nickel  on  the  critical  points  of 
iron.     (Osmond.) 


other  words,  nickel  up  to  25  per  cent  greatly  increases  the  hysteresis.  Taking  a  steel, 
for  instance,  with  10  per  cent  nickel  cooling  from  a  high  temperature,  it  remains  non- 
magnetic until  a  temperature  of  some  400  deg.  C.  is  reached  when  it  undergoes  the 
magnetic  and  other  transformations.  On  reheating  this  magnetic  steel,  however,  it 
does  not  again  lose  its  magnetism  until  a  temperature  of  some  675  deg.  is  attained. 
Between  400  and  675  deg.  this  nickel  steel  will  be  magnetic,  therefore,  in  case  its  last 
transformation  resulted  from  cooling  below  400  deg.  and  it  will  be  non-magnetic  if  it 
resulted  from  heating  above  675  degrees.  When  this  hysteresis  gap  between  the 
transformation  is  considerable  the  alloys  are  said  to  be  irreversible,  meaning  by  that 
expression  that  the  reverse  transformation  cannot  be  produced  at  or  near  the  same 
temperature.  Nickel  steels  containing  between  5  and  25  per  cent  nickel  are  therefore 
often  spoken  of  as  irreversible  alloys.  It  should  be  noted,  that  when  the  nickel 
content  does  not  exceed  some  3  per  cent  the  alloys  are  really  reversible,  that  is,  the 
gap  between  the  critical  transformations  on  heating  and  cooling  is  not  excessive. 
According  to  Osmond,  for  instance,  with  3.82  per  cent  nickel  the  critical  point  on 
heating  occurs  at  710  deg.  and  on  cooling  at  628  degrees.  A  gap  of  100  deg.  might  b  e 


338 


CHAPTER  XXI  — ALLOY  STEELS 


arbitrarily  selected  as  a  line  of  demarcation  between  reversible  and  irreversible  alloys. 
Returning  to  Figure  310  it  will  be  seen  that  as  the  nickel  content  increases  above  25 
per  cent  the  transformations  become  abruptly  reversible  (the  gap  between  them  not 
exceeding  50  deg.),  that  their  position  is  now  gradually  lifted,  reaching  a  maximum 
for  about  70  per  cent  nickel,  and  that  it  is  then  again  lowered.  Iron-nickel  alloys 
containing  more  than  25  per  cent  nickel  are  therefore  reversible. 

The  diagram  also  shows  that  with  some  25  per  cent  nickel  the  transformation  is 
lowered  below  atmospheric  temperature  which  means  that  the  metal  on  cooling  from 
above  B'  remains  non-magnetic  at  atmospheric  temperature  and  that  its  iron,  there- 
fore, is  in  the  gamma  condition  and  its  structure  austenitic. 


75  10.  IZ.5          15.  17.5          20.  22.5          25 


Nickel  %        0  23  5. 


Fig.  311.  —  Influence  of  nickel  and  carbon  on  the  position  of  the  critical  point  An  and  corresponding 

types  of  structure. 


An  attempt  has  been  made  in  Figure  311  to  construct  a  diagram  indicating  the  re- 
lation existing  between  carbon  content,  nickel  content,  position  of  the  critical  points 
on  cooling,  and  corresponding  types  of  structures  as  explained  in  Chapter  XX. 

As  already  stated  the  pearlitic  nickel  steels  are  those  most  widely  used.  In  the 
majority  of  cases  the  nickel  content  does  not  exceed  3.50  per  cent  while  the  carbon 
content  is  seldom  over  0.50  per  cent.  These  steels  compared  with  carbon  steels  of 
equal  ductility  have  a  considerably  higher  strength  and  especially  higher  elastic 
limit,  while  compared  with  carbon  steels  of  like  elastic  limit  they  have  much  greater 
ductility.  To  explain  this  in  another  way,  the  introduction  of  some  3.50  per  cent 
nickel  in  a  0.50  per  cent  carbon  steel,  for  instance,  raises  its  elastic  limit  very  con- 
siderably while  decreasing  its  ductility  but  slightly.  Pearlitic  nickel  steels  are  also 
somewhat  harder  than  carbon  steels  of  like  properties,  hence  better  able  to  resist  wear. 
When  properly  heat  treated  their  ability  to  resist  shocks  is  likewise  greater. 


CHAPTER   XXI  — ALLOY   STEELS 


339 


The  structure  of  pearlitic  nickel  steel  is  shown  in  Figure  312.  On  comparing  it 
with  that  of  carbon  steel  of  like  carbon  content  it  will  be  noted  that  the  pearlite  par- 
ticles are  somewhat  sharper  and  more  angular  and  the  ferrite  grains  smaller.  When 
examined  under  high  magnification  the  nickel  pearlite  is  seldom  found  as  distinctly 
laminated  as  ordinary  pearlite. 

The  hardening  and  annealing  of  nickel  steels  should  be  conducted  at  lower  tem- 
peratures than  the  hardening  and  annealing  of  ordinary  steels  of  similar  carbon  con- 
tent since  their  critical  points  occur  at  lower  temperatures.  From  the  evidences  at 
hand  it  would  seem  as  if  between  0  and  5  per  cent  nickel,  and  in" the  case  of  low  car- 
bon steels,  each  one  per  cent  of  nickel  lowered  the  Ari  point  some  20  deg.  C.  and  the 


*^l'^/M*% 

4&&:i&&&M 


i*.  si^^y^  a^s 


Fig.  312.  —  Nickel  steel.  Carbon  about  0.30  per  cent. 
Nickel  about  3  per  cent.  Magnified  100  diameters. 
(G.  A.  Reinhardt  in  the  author's  laboratory.) 


Aci  point  some  10  degrees.  In  the  nickel  pearlitic  steels  of  commerce,  therefore,  the 
points  Ari  and  Aci  should  occur  at  or  near  the  temperatures  indicated  in  the  follow- 
ing table  according  to  their  percentage  of  nickel. 


0 

0.50 

1.00 

1.50 

2.00 

2.50 

3.00 

3.50 

4.00 

4.50 

5.00 


750 
745 
740 
735 
730 
725 
720 
715 
710 
705 
700 


A* 
700 
690 
680 
670 
660 
650 
640 
630 
620 
610 
600 


340 


CHAPTER   XXI  — ALLOY   STEELS 


Fig.  313.  —  Nickel  steel.  Nickel  3.44  per  cent.  Carbon  0.176  per  cent.  Case 
hardened  and  air  cooled.  Magnified  100  diameters.  (G.  A.  Reinhardt  in 
the  author's  laboratory.) 


Fig.  314.  —  Same  steel  as  in  Figure  313.    Same  treatment.    Magnified  50  diam- 
eters.    (G.  A.  Reinhardt  in  the  author's  laboratory.) 


CHAPTER   XXI— ALLOY   STEELS 


341 


Fig.  315.  —  Nickel  steel.  Nickel  4.86  per  cent.  Carbon  0.115  per  cent.  Case 
hardened  and  air  cooled.  Magnified  100  diameters.  (G.  A.  Reinhardt  in  the 
author's  laboratory.) 


Fig.  316.  —  Same  steel  as  in  Figure  315.    Same  treatment.    Magnified  300  diam- 
eters.    (G.  A.  Reinhardt  in  the  author's  laboratory.) 


342 


CHAPTER  XXI  — ALLOY   STEELS 


While  nickel  retards  the  carburization  of  iron  by  case  hardening,  the  cores  of 
nickel  steel  articles  are  not  coarsened  by  the  high  temperature  of  the  carburizing 
operation  to  the  same  extent  as  carbon  steel  cores,  so  that  one  treatment  is  often 
sufficient,  namely,  reheating  to  and  quenching  from  a  temperature  slightly  superior 
to  the  critical  range  of  the  case,  that  is,  to  some  700  to  750  deg.  in  the  presence  of 
some  3  or  3.5  per  cent  nickel.  Higher  nickel  contents  call  for  lower  quenching  tem- 
peratures. 

The  case  hardening  of  nickel  steels  offers  the  possibility  already  alluded  to  of 
producing  a  martensitic  case  without  quenching.  Nickel  steel,  for  instance,  contain- 
ing not  over  0.25  per  cent  carbon  and  some  3.50  or  more  per  cent  nickel  can  readily 
be  made  martensitic  near  the  outside  by  case  hardening  followed  by  air  cooling  as 


Fig.  317.  —  Nickel  steel.  Cast.  Nickel  10 
percent.  Carbon  0.80  per  cent.  Magnified 
300  diameters.  (Guillet.) 


Fig.  318.  —  Nickel  steel.  Nickel  25  per 
cent.  Carbon  0.80  per  cent.  Magnified  300 
diameters.  (Osmond.) 


shown  in  Figures  313  and  314.  The  martensitic  grains  owe  their  polyhedral  form  to 
the  original  austenitic  grains  from  which  they  are  derived.  The  thickness  of  the  mar- 
tensitic case  is  about  0.5  mm.  The  occurrence  of  troostite  should  be  noted.  Under 
lower  magnification  (Fig.  314)  a  solid  troostite  band  is  seen  to  separate  the  marten- 
sitic and  the  sorbito-pearlitic  portions.  With  a  little  more  carbon  and  nickel  mar- 
tensito-austenitic  cases  may  be  produced  as  shown  in  Figures  315  and  316. 

Nickel  steels  that  are  martensitic  as  cast  are  not  utilized  because  like  all  mar- 
tensitic steels  they  are  hard,  brittle,  and  cannot  be  machined.  Their  case  hardening 
should  result  in  the  formation  of  ductile,  austenitic  cases.  Nickel  steels  which  are 
martensitic  after  air  cooling  may  be  troostitic,  sorbitic,  or  even  pearlitic  after  very 
slow  cooling  in  the  furnace,  while  they  may  become  austenitic  on  water  quenching. 
The  structure  of  martensitic  nickel  steel  is  shown  in  Figure  317. 

Austenitic  nickel  steels  are  not  widely  used,  the  high  carbon,  high  manganese 
steels  being  preferred  when  an  austenitic  steel  is  desired,  in  part  at  least  because  of 
their  lower  cost.  Like  all  austenitic  steels  they  are  non-magnetic,  ductile,  very  diffi- 
cult to  machine,  and  have  a  low  elastic  limit,  Their  structure  is  polyhedral  (see 


CHAPTER  XXI  — ALLOY   STEELS  343 

Fig.  318).  Some  types  of  austenitic  nickel  steels  have,  however,  found  interesting 
applications  based  chiefly  on  the  marked  influence  of  nickel  on  the  dilatation  of  the 
metal.  With  36  per  cent  nickel,  for  instance,  the  dilatation  is  nearly  nil  and  the  re- 
sulting alloy,  discovered  by  Guillaume,  and  called  by  him  "invar"  is  used  success- 
fully for  the  construction  of  clocks  and  other  instruments  of  precision.  With  some 
46  per  cent  of  nickel  and  0.15  per  cent  carbon  the  coefficient  of  dilatation  is  nearly 
the  same  as  that  of  glass  and  alloys  of  that  composition  called  "platinite"  are  used 
in  place  of  platinum  for  the  construction  of  incandescent  electric  lamps.  Austenitic 
nickel  steel  may  be  made  martensitic  and  thereby  regain  its  magnetism  by  immer- 
sion in  liquid  air.  The  increase  of  volume  which  accompanies  this  transformation 
produces  a  swelling  of  the  polished  surface  which  because  of  the  resulting  relief  effect 
renders  the  structure  of  the  metal  apparent  without  etching,  as  shown  in  Figure  319. 


Fig.  319.  —  Nickel  steel.  Nickel  15  per 
cent.  Carbon  0.80  per  cent.  Cooled  in 
liquid  air  (-180  deg.  C.).  Not  etched. 
Magnified  300  diameters.  (Guillet.) 

Manganese  Steel.  —  Manganese,  when  alloyed  with  iron  and  carbon  in  large 
proportion,  is  partly  dissolved  in  the  iron  and  partly  present  as  a  double  carbide  of 
iron  and  manganese.  From  this  behavior  of  manganese  the  structural  types  formed 
by  increasing  both  carbon  and  manganese  may  be  anticipated.  The  steel  should  at 
first  remain  pearlitic  and  then  become  in  succession  martensitic  and  austenitic.  With 
much  manganese  and  carbon,  however,  the  separation  of  carbide  is  to  be  expected. 
The  constitutional  diagram  of  manganese  steels  is  shown  in  Figure  320  after  Guillet, 
while  a  critical  point  structural  diagram  has  been  constructed  tentatively  in  Figure 
321.  By  comparing  the  constitutional  diagram  of  manganese  steel  with  that  of  nickel 
steel  it  will  be  noted  that  manganese  is,  roughly  stated,  twice  as  effective  as  nickel 
in  producing  a  certain  type  of  structure,  as  for  instance  in  converting  pearlitic  into 
martensitic  steel. 

Manganese  is  present  in  appreciable  quantities  in  all  ordinary  carbon  steels  but 
unless  the  latter  contain  considerably  more  than  one  per  cent  of  that  element  they 
are  not  regarded  as  manganese  steels.  With  carbon  not  exceeding  0.80  per  cent  and 


344 


CHAPTER  XXI  — ALLOY  STEELS 


manganese  not  exceeding  some  3  per  cent  the  steels  remain  pearlitic  and,  therefore, 
not  unlike  the  pearlitic  nickel  steels  so  widely  used.  Manganese  pearlitic  steels, 
however,  are  practically  ignored  by  steel  manufacturers  and  users  apparently  (1)  be- 
cause of  the  wide-spread  belief  that  such  steels  are  brittle  and  (2)  because  of  the 
difficulty  of  manufacturing  low  carbon  manganese  steels.  The  belief  in  the  brittle- 
ness  of  pearlitic  manganese  steels  is  founded  on  Hadfield's  statement  that  between 
2  and  6  per  cent  of  manganese  the  steels  are  hopelessly  brittle.  On  closer  examina- 
tion, however,  it  would  seem  as  if  this  statement  was  true  only  in  the  case  of  rather 
high  carbon  steels  cooled  relatively  quickly.  Evidences  have  since  been  offered, 


AT 


<o 
Q) 
C 


/oj 


Ausfenific 


Morten  si  fie 


Pearl  if  i  c 


0  0,4-  O.Q  1.2. 

%  Carbon 

Fig.  320.  —  Manganese  steel.     Constitutional  diagram.     (Guillet.) 


t.e 


notably  by  Guillet,  showing  that  low  carbon  pearlitic  manganese  steel  slowly  cooled  is 
not  brittle.1  These  steels  have  been  difficult  to  manufacture  because  of  the  necessity 
of  using  ferro-manganese  from  the  blast  furnace  and  therefore  high  in  carbon,  thereby 
introducing  much  carbon  in  the  steel.  At  present,  however,  nearly  carbonless  ferro- 
manganese  is  produced  in  electric  furnaces  and  also  by  the  thermit  process.  It  is 
also  claimed  that  low  carbon  manganese  steels  can  be  successfully  produced  in  the 
electric  furnace  under  suitable  oxidizing  conditions  and  at  high  temperature  when 
carbon  may  be  oxidized  in  preference  to  manganese.  If  the  physical  properties  of 
low  carbon  pearlitic  manganese  steel  are  at  all  comparable  to  those  of  pearlitic  nickel 
steel  and  if  manganese  steel  can  be  manufactured  more  cheaply  than  nickel  steel  of 

1  Arnold  reports  that  steel  containing  less  than  0.10  per  cent  carbon  and  5.50  per  cent  manganese 
has  a  tensile  strength  of  some  145,000  pounds  per  square  inch  and  an  elongation  of  28.50  per  cent. 


CHAPTER  XXI  — ALLOY   STEELS 


345 


like  properties  the  manufacture  and  testing  of  low  carbon  manganese  steel  should 
receive  more  attention.  The  possibility  of  producing  by  case  hardening  articles  hav- 
ing pearlitic  cores  and  austenitic  cases  also  deserves  some  consideration.  Marten- 
sitic  manganese  steels  are  not  utilized  because  of  the  hardness  and  brittleness  which 
they  share  with  all  martensitic  steels.  Those  manganese  steels  whose  composition 
is  near  the  boundary  line  between  the  pearlitic  and  martensitic  regions  while  marten- 
sitic after  air  cooling  may  be  troostitic  or  even  pearlitic  after  very  slow  cooling  while 
they  may  become  austenitic  on  quenching. 

Austenitic  manganese  steel  is  of  considerable  industrial  importance.    It  is  often 
called  from  the  name  of  its  inventor  "Hadfield"  steel.    It  generally  contains  from 


700 


%Mn 


5.  7.5 

Austenitic 


Fig.  321.  — 'Influence  of  manganese  and 
carbon  on  position  of  critical  point  An 
and  corresponding  types  of  structure. 


10  to  15  per  cent  manganese  and  from  one  to  1.5  per  cent  carbon.  In  its  cast  condi- 
tion it  is  weak  and  has  but  little  ductility  probably  because  of  the  presence  of  a  con- 
siderable quantity  of  free  carbide.  On  being  heated  to  a  high  temperature  (1000 
deg.  C.  or  more),  however,  and  quenched  in  water  or  oil  its  tenacity  is  greatly  raised 
and  it  becomes  very  ductile;  the  treatment  being  often  called  on  that  account  "water 
toughening."  The  marked  change  of  properties  resulting  from  it  is  probably  due  to 
the  absorption  of  the  carbide  at  a  high  temperature  and  its  retention  in  solution  by 
quick  cooling.  The  structure  of  manganese  steel  both  in  its  cast  and  in  its  water 
quenched  condition  is  shown  in  Figures  322  to  325.  In  the  cast  sample  the  carbide 
is  seen  to  occur  as  thick  membranes  surrounding  the  austenitic  grains  and  here  and 
there  in  chunks.  The  treated  sample  is  nearly  free  from  carbide  and  possesses  the 
polyhedral  structure  characteristic  of  gamma  iron  and  of  austenite.  The  properties 


346 


CHAPTER  XXI  — ALLOY   STEELS 


of  austenitic  manganese  steel  are  those  of  austenite,  namely  low  elastic  limit  but 
great  hardness  and  wearing  power  combined  with  much  ductility. 

Tungsten  Steels.  —  Tungsten  appears  to  raise  rather  than  lower  the  critical 
points  of  iron  while  it  forms  with  it  a  double  carbide  of  iron  and  tungsten  from  which 


.  ^j 


'  . 


Fig.  322.  —  Manganese  steel.     Austenitic.     Cast.     Magnified  50  diameters. 


Fig.  323.  —  Same  as  in  Figure  322.      Magnified         Fig.   324.  —  Manganese  steel.       Austenitic. 
300  diameters.  Water  quenched.     Magnified  100  diameters. 

it  may  be  safely  inferred  that  tungsten  steels  will  at  first  remain  pearlitic  on  slow 
cooling  and  that  as  the  percentage  of  tungsten  increases  it  will  become  cementitic, 
that  is  it  will  contain  carbide  particles.  This  is  shown  in  Figure  326,  which  is  a  re- 
production of  the  constitutional  diagram  of  tungsten  steels  according  to  Guillet.  In 


CHAPTER   XXI  — ALLOY   STEELS 


347 


' 


-AJJ 


Fig.  325.  —  Manganese  steel.    Austenitic.    Water  quenched.    Mag- 
nified 100  diameters. 


.  a 


<0 


O 


Cemenf/ti  c 


Pearl  i 'fie 


•)  04-  0.<3  1.2 

°/o  Carbon 

Fig.  326.  —  Tungsten  steel.     Constitutional  diagram.     (Guillet.) 


i.e 


348 


CHAPTER   XXI  — ALLOY   STEELS 


the  presence  of  a  considerable  proportion  of  tungsten,  however,  the  position  of  the 
Ari  point  seems  to  be  greatly  affected  by  the  temperature  from  which  the  steel  cools. 
Osmond,  for  instance,  found  in  the  case  of  a  steel  containing  0.42  per  cent  carbon  and 
6.25  per  cent  tungsten  that  heating  to  and  cooling  from  some  900  deg.  reveals  the 
existence  of  two  critical  points  respectively  at  690  and  650  deg.  As  the  temperature 
from  which  cooling  begins  increases,  however,  two  interesting  phenomena  are  ob- 
served, (1)  the  upper  point  occurs  at  practically  the  same  temperature  but  becomes 
fainter  and  finally  disappears  and  (2)  the  lower  point  remains  pronounced  but  its 
position  is  gradually  lowered.  Cooling  from  1015  deg.,  for  instance,  resulted  in  a  faint 
critical  point  at  670  deg.  while  the  lower  point  was  depressed  to  625  deg. ;  cooling  from 
1210  deg.  caused  the  disappearance  of  the  upper  point  while  the  lower  remained  very 
pronounced  but  now  occurred  at  500  degrees.  Bohler,  likewise  experimenting  with  a 
steel  containing  0.85  per  cent  carbon  and  7.78  per  cent  tungsten,  reports  the  exist- 


Fig.  327.  —  Tungsten  steel.  Tungsten  27.75 
per  cent.  Carbon  0.276  per  cent.  Mag- 
nified 200  diameters.  (Guillet.) 


Fig.  328.  —  Tungsten  steel.  Tungsten  39.96 
per  cent.  Carbon  0.867  per  cent.  Mag- 
nified 200  diameters.  (Guillet.) 


ence  of  a  point  at  710  deg.  and  one  at  550  deg.,  the  upper  one,  however,  occurring 
only  when  the  metal  has  not  been  heated  above  1 100  deg.  while  the  second  is  only  to 
be  detected  when  the  temperature  exceeds  1000  deg.  In  other  words  on  cooling  from 
above  1100  deg.  the  lower  point  only  occurs,  on  cooling  from  below  1000  deg.  only 
the  upper  point  is  visible,  while  heating  to  and  cooling  from  a  temperature  situated 
between  1000  and  1100  deg.  causes  the  appearance  of  both  points.  It  will  be  shown 
soon  that  this  indirect  influence  of  tungsten  upon  the  critical  points  affords  an  ex- 
planation of  the  remarkable  properties  of  self-hardening  and  high  speed  steels  which 
are  chiefly  tungsten  steels. 

On  heating  cementitic  tungsten  steels  to  a  high  temperature  the  particles  of  free 
carbides  are  dissolved  the  more  completely  the  higher  the  temperature.  Air  cooling 
is  often  sufficient  to  retain  the  carbides  in  solution  while  the  metal  becomes  finely 
martensitic.  Such  steels  are  said  to  be  "self -hardening."1  The  structure  of  two 

1  The  presence  of  manganese  or  of  a  little  chromium  is  necessary,  however,  to  impart  self-hard- 
ening properties  to  tungsten  steel.  The  original  "self"  or  "air-hardening"  steel,  that  is  "Mushet" 
steel,  always  contained  considerably  more  than  one  per  cent  manganese  and  was  high  in  carbon. 


CHAPTER  XXI  — ALLOY   STEELS  349 

cementitic  tungsten  steels  is  reproduced  in  Figures  327  and  328.  The  white  particles 
are  the  double  carbide. 

Tungsten  steels  are  used  (1)  for  springs  generally  after  hardening  followed  by 
tempering,  (2)  for  magnets  after  hardening  only,  and  (3)  for  tools  as  self-hardening 
steels.  In  the  latter  case,  however,  considerable  manganese  is  always  present,  the 
resulting  alloy  being  in  reality  a  quaternary  steel.  High  speed  steels  are  quaternary 
steels  generally  containing  a  large  proportion  of  tungsten;  they  will  soon  be  described. 

Chrome  Steels.  -  -  Chromium  forms  double  carbides  with  iron  probably 
xFe3C.yCr3C2  or  possibly  solid  solutions  of  the  two  carbides,  while  Tt  has  little  if  any 
direct  influence  on  the  position  of  the  critical  points.1  The  presence  of  chromium, 
however,  like  that  of  tungsten  causes  the  point  on  cooling  to  be  markedly  lowered  as 
the  temperature  from  which  the  metal  cools  increases.  Osmond,  for  instance, 
found  the  following  relations  between  the  maximum  temperature  and  the  position  of 
the  critical  point  on  cooling: 

MAXIMUM  CRITICAL  POINT  MAXIMUM  CRITICAL  POINT 

TEMPERATURE  ON  COOLING  TEMPERATURE  ON  COOLING 

835  713-716  1220  635-643 

1030  682-692  1320  600-640 

The  constitutional  diagram  of  chrome  steels  is  shown  in  Figure  329  after  Guillet. 
Reheating  cementitic  chrome  steel  to  a  high  temperature  followed  by  quick  cooling 
(air  or  water)  results  generally  in  the  disappearance  of  some  of  the  particles  of  free 
carbide.  The  case  hardening  of  chrome  steels  yields  very  hard  cases.  As  the  core 
coarsens,  however,  these  steels  should  always  receive  the  double  treatment  described 
in  Chapter  XIX,  that  is,  (1)  reheating  and  quenching  for  refining  the  core  and  (2)  re- 
heating and  quenching  for  refining  and  hardening  the  case. 

The  chrome  steels  that  are  utilized  seldom  contain  more  than  3  per  cent  chromium 
and  are  therefore  pearlitic  after  slow  cooling;  they  are  used  for  the  manufacture  of 
armor  piercing  projectiles,  of  steel  balls,  of  files,  and  of  some  other  tools,  the  presence 
of  chromium  increasing  the  hardness  and  the  hardening  power  of  the  metal. 

Becker  writes  that  the  hardness  imparted  by  chromium  is  not  accompanied  by  as 
much  brittleness  as  that  induced  by  carbon.  According  to  the  same  author  chromium 
also  has  the  effect  of  increasing  the  elastic  limit  of  steel,  especially  when  it  is  com- 
bined with  vanadium.  The  structure  of  pearlitic  chromium  steel  resembles  that  of 
pearlitic  carbon  steel. 

Vanadium  Steels.  —  Vanadium  forms  double  carbides2  with  iron  and  has  no 
marked  influence  on  the  positions  of  the  critical  points.  Unlike  tungsten  and  chro- 

1  According  to  some  recent  observations  of  Nesselstrauss,  chromium  lowers  the  point  Ar3  of 
hypo-eutectoid  steels,  eventually  causing  its  disappearance  while  it  raises  a  little  the  point  Art.    The 
proportion  of  chromium  needed  to  cause  the  point  Ar3  to  disappear  is  the  smaller,  the  higher  the 
carbon  content.    With  0.20  per  cent  carbon,  5  per  cent  chromium  are  needed.    With  more  carbon  a 
correspondingly  smaller  percentage  of  chromium  suffices.     Recent  experiments  conducted  at  the 
Watertown  Arsenal,  Watertown,  Mass.,  with  steel  containing  0.50  per  cent  carbon,  also  indicate 
that  increasing  the  chromium  content  from  0  to  4.05  per  cent  raises  the  Aci  point  from  743  to 
791  deg.  C.  and  the  Ar:  point  from  671  to  748  deg.  C. 

2  Arnold  and  Read  believe  that  mixtures  of  carbide  of  iron  (Fe3C)  and  of  carbide  of  vanadium 
(V.iC3)  are  formed  rather  than  double  carbides.    In  alloys  containing  6  per  cent  vanadium  the  carbide 
formed  contains  only  vanadium  according  to  these  writers. 


350 


CHAPTER  XXI  — ALLOY   STEELS 


E 

o 


Pear  I  it  ic 


o.Q  1.2 

%  Carbon . 

Fig.  329.  —  Chrome  steel.     Constitutional  diagram.     (Guillet.) 


O 


Oeme n  //  f  i  c 


^ 

Pear  lit ic. 


O  O.S  1.0  1.5 

%Carbon 

Fig.  330.  —  Vanadium  steel.     Constitutional  diagram.     (Guillet.) 


2.0 


CHAPTER   XXI  — ALLOY   STEELS 


351 


mium  it  does  not  seem  to  cause  the  lowering  of  the  Ar  points  with  increasing  tem- 
perature. The  constitutional  diagram  of  vanadium  steels  is  shown  in  Figure  330 
after  Guillet.  Two  types  of  structures  are  produced,  pearlitic  and  cementitic.  Guillet 
describes  the  appearance  of  the  particles  of  free  carbide  as  being  triangular. 

According  to  Guillet  heating  cementitic  vanadium  steels  to  &•  high  temperature 
fails  to  cause  the  absorption  of  the  free  carbide,  vanadium  steels  differing  in  this  re- 
spect from  other  steels  in  which  double  carbides  are  formed. 

The  vanadium  steels  commercially  utilized  seldom  contain  more  than  one  per 
cent  vanadium  and  are,  therefore,  pearlitic  and  free  from  carbides.  Their  proper- 


OJ 


O 


2,5 


Cementitic 


Pearlitic 


0,5 


1,5    1,6 


Z Carbon 

Fig.  331.  —  Molybdenum  Steel.     Constitutional  diagram.     (Guillet.) 


ties  recall  those  of  pearlitic  nickel  steels,  namely  high  combination  of  elastic  limit  and 
ductility  and  greater  resistance  to  shock,  to  alternate  stresses,  and  to  wear.  The  very 
small  amount  of  vanadium  sufficient  to  produce  these  results  should  be  noted. 

Molybdenum  Steels.  —  The  constitutional  diagram  of  molybdenum  steels  is 
shown  after  Guillet  in  Figure  331.  With  increasing  carbon  and  molybdenum,  the 
steels  which  were  at  first  pearlitic  on  slow  cooling  become  cementitic.  According  to 
Thomas  Swinden  the  Ari  point  of  these  steels  is  steadily  lowered  as  the  temperature 
from  which  cooling  starts  increases,  that  lowering  being  the  more  marked  the  larger 
the  percentages  of  molybdenum  and  carbon.  The  same  author  writes  that  molybde- 
num renders  the  pearlite  very  emulsified,  even  in  annealed  steel;  that  it  lowers  the 
saturation  point,  with  8  per  cent  molybdenum  from  0.36  to  0.44  per  cent  carbon 
being  sufficient  for  the  exclusion  of  free  ferrite;  that  any  treatment  involving  slow 
cooling  coarsens  the  structure  and  is  very  detrimental;  that  the  chemical  residue  of 


352 


CHAPTER  XXI  — ALLOY   STEELS 


annealed  molybdenum  steel  contains  91.7  per  cent  of  the  carbon  present  in  the  steel, 
89.5  per  cent  of  the  molybdenum,  and  generally  more  iron  than  is  required  by  the 
formula  FesC,  from  which  it  is  inferred  that  molybdenum  is  not  combined  with  carbon 
but  is  present  as  an  iron-molybdenum  compound,  and  that  there  is  no  increase  in 
hardness  as  molybdenum  increases  from  1  to  8  per  cent. 

According  to  Guillet  molybdenum  has  an  influence  similar  to  that  of  tungtsen 
on  the  physical  properties  and  on  the  structure  of  steel  but  four  times  more  intense. 

Silicon  Steels.  —  Silicon,  probably  as  an  iron  silicide,  FeSi,  forms  a  solid  solution 
with  iron  in  all  proportions  and  has  no  very  marked  influence  upon  the  position  of  the 

30 . 


2Q 


C 
o 
o 


10. 


G  ra  ph  i  fe  +Fe  Si 


Grap kite  -f-so/uf/on  (Fe Si+ 


Pear/lte+graphite  +  Solution  FeSi. 


Pearl ite+solufj on 


0.5-  1,0  1.5 

%  Carbon 

Fig.  332.  —  Silicon  steel.     Constitutional  diagram.     (Guillet.) 


20 


critical  points  from  which  we  may  infer  that  slowly  cooled  silicon  steels  will  be  neither 
martensitic  nor  cementitic.  It  is  a  well-known  fact,  moreover,  that  silicon  has  a 
marked  tendency  to  cause  the  formation  of  graphitic  carbon  when  present  over  a 
certain  proportion  especially  in  high  carbon  steel.  The  constitutional  diagram  of 
silicon  steels  is  shown  in  Figure  332  according  to  Guillet.  It  will  be  seen  that  the 
structure  is  independent  of  the  carbon  content  being  entirely  regulated  by  the  pro- 
portion of  silicon.  As  long  as  the  proportion  of  silicon  does  not  exceed  5  per  cent  the 
steel  is  pearlitic  and  the  whole  of  the  carbon  remains  in  the  combined  condition. 
Between  5  and  7  per  cent  of  silicon  some  pearlite  is  still  present  and,  hence,  some  com- 
bined carbon,  but  graphitic  carbon  also  occurs;  between  7  and  20  per  cent  of  silicon 
the  whole  of  the  carbon  is  in  the  graphitic  condition,  the  balance  of  the  steel  consist- 
ing of  a  solid  solution  of  the  silicide  FeSi  in  iron  (silico-ferrite),  and  also,  according  to 


CHAPTER   XXI  — ALLOY   STEELS  353 

( Uiillet,  of  some  FesSi  likewise  dissolved  in  iron.  With  more  than  20  per  cent  of  sili- 
con the  steel  is  composed  of  graphite  and  of  FeSi. 

It  should  be  noted  that  the  only  silicon  steels  utilized  contain  less  than  5  per  cent 
of  silicon  and  are,  therefore,  pearlitic  and  free  from  graphitic  carbon  unless  indeed 
subjected  to  prolonged  annealing  or  very  slow  cooling  when  some  graphitic  carbon 
will  form,  especially  in  high  carbon  steels.  This  formation  of  graphitic  carbon  takes 
place  the  more  readily  the  higher  the  temperature,  the  longer  the  time  at  a  high  tem- 
perature, the  more  silicon  and  the  more  carbon  in  the  steel. 

Silicon  steels  are  chiefly  used  in  the  construction  of  dynamos  Jjecause  of  their 
low  magnetic  hysteresis  and  high  permeability  and,  with  the  addition  of  quite  a  little 
manganese,  for  springs  and  for  certain  parts  of  automobiles. 

Chrome-Nickel  Steels.  —  From  our  knowledge  of  the  constitution  of  nickel  steels 
and  of  chromium  steels  it  is  possible  to  foretell  the  constitution  of  the  quaternary 
chrome-nickel  steels.  Nickel  steels  being  pearlito-martensito-austenitic  and  chrome 
steels  being  chiefly  pearlito-cementitic,  we  may  expect  that  in  chrome-nickel  steels, 
as  the  proportions  of  carbon,  nickel,  and  chromium  increase  the  steels,  at  first  pear- 
litic, will  become  in  turn  martensitic,  austenitic,  and  cementitic.  In  the  case  of  cem- 
entitic  steels  the  free  carbides  will  be  embedded  in  a  martensitic  or  austenitic  matrix 
according  to  their  composition.  It  should  also  be  expected  that  increasing  the  pro- 
portion of  nickel  (and  carbon)  will  cause  the  steel  to  pass  from  the  pearlitic  to  the 
martensitic  condition,  etc.,  while  increasing  the  chromium  (and  carbon)  will  make  it 
cementitic.  The  presence  of  both  nickel  and  chromium  in  the  same  steel  produces 
a  metal  possessing  the  valuable  qualities  of  both  nickel  and  chromium  steels,  namely, 
high  elastic  limit  combined  with  high  ductility,  greater  hardness,  hardening  power, 
resilience,  and  better  wearing  qualities  than  carbon  steels.  Chrome-nickel  steels  are 
especially  valuable  in  the  construction  of  parts  to  be  hardened  and  tempered  when 
they  yield  a  finely  martensitic  structure  having  greater  shock  resisting  power  than  the 
martensite  of  carbon  steels.  Practically  the  only  chrome-nickel  steels  utilized  are  the 
pearlitic  ones  containing  therefore  moderate  amounts  of  carbon,  nickel,  and  chro- 
mium. They  are  used  extensively  in  automobile  construction  and  for  the  manufac- 
ture of  armor  plates.  In  the  latter  case,  of  course,  one  face  of  the  plates  is  subjected 
to  the  case  hardening  treatment.  The  case  hardening  of  nickel-chromium  steel  re- 
sembles that  of  nickel  steel.  The  metal  should  be  reheated,  after  the  case  hardening 
operation,  slightly  above  the  critical  range  of  the  case  and  quenched.1  Because  of 
the  presence  of  nickel  it  is  not  so  imperative  to  heat  to  and  quench  from  a  tempera- 
ture superior  to  the  critical  range  of  the  core  before  hardening  the  case,  although 
such  procedure  would  probably  yield  a  tougher  core. 

Quaternary  Vanadium  Steels.  —  The  introduction  of  a  small  amount  of  vana- 
dium into  the  various  special  steels  has  been  strongly  urged  and  nickel-vanadium, 
chrome-vanadium,  chrome-nickel-vanadium,  and  chrome-tungsten-vanadium  steels 
have  been  used.  It  is  claimed  that  the  presence  of  a  small  proportion  of  vanadium 
(less  than  0.50  per  cent)  increases  the  soundness  of  castings  and  their  freedom  from 
occluded  gases  and  that  it  adds  to  the  desirable  physical  qualities  of  forgings  such  as 
strength,  resilience,  ductility,  etc.  Guillet  writes  that  nickel-vanadium  steel  prop- 
erly hardened  by  quenching  is  relatively  so  tough  that,  unlike  other  hardened  steels, 

1  Grenet  states  that  the  quenching  of  chrome-nickel  steel  may  be  done  in  boiling  water  satu- 
rated with  salt,  thus  avoiding  the  possible  distortions  of  more  drastic  treatment,  while  the  metal  is 
made  very  hard. 


354  CHAPTER  XXI  — ALLOY   STEELS 

it  does  not  require  tempering.  Since  vanadium  forms  a  double  carbide  with  iron  its 
presence  in  steel  is  likely  to  make  it  cementitic.  In  nickel  steel  and  in  nickel- 
chromium  steel  the  matrix  will  be  pearlitic,  martensitic,  or  austenitic  in  accordance 
with  the  proportion  of  nickel  and  carbon  present ;  in  chrome  steels  it  will  be  pearlitic 
or  martensitic. 

Chrome-Tungsten  or  High  Speed  Steels.  —  Slowly  cooled  chrome-tungsten  steels 
are  cementitic  with  a  pearlitic,  sorbitic,  or  even  troostitic  matrix.  Upon  being 
heated  to  a  high  temperature  the  carbide  particles  are  dissolved,  at  least  for  the 
most  part,  and  if  the  cooling  that  follows  be  sufficiently  rapid  they  are  retained  in 


Fig.  333.  —  High  speed  steel.  Quenched  in  air  blast  at  1160  deg.  C. 
Magnified  500  diameters.  Tungsten  about  13  per  cent.  Chro- 
mium about  4  per  cent.  Carbon  0.46  per  cent.  (H.  B.  Pulsifer.) 

solution,  the  metal  acquiring  a  finely  austenitic,  martensitic,  or  austenito-martensitic 
structure.  To  cause  a  complete  absorption  of  the  free  carbide,  however,  a  very 
high  temperature  is  often  required,  in  some  cases  approaching  the  melting-point 
of  the  steel,  while  air  cooling  is  frequently  sufficiently  rapid  to  prevent  the  carbide 
from  again  forming.  After  such  treatment  these  steels  although  fully  hardened 
are  in  a  condition  relatively  so  stable  that  they  may  be  heated  to  a  visibly  red 
heat,  i.e.  to  some  600  deg.  C.  before  undergoing  any  marked  transformation. 
This  invaluable  property  makes  it  possible,  with  tools  made  of  such  steels  suitably 
treated,  to  cut  steel  and  other  hard  metals  at  such  speed  that  the  cutting  edge  of  the 
tool  becomes  visibly  red  hot  before  breaking  down.  These  steels  are  known  in  con- 
sequence as  high  speed  steels.  Their  discovery  by  Taylor  and  White,  at  the  time  in 


CHAPTER   XXI  — ALLOY   STEELS  355 

the  employ  of  the  Bethlehem  Steel  Company,  South  Bethlehem,  Pennsylvania, 
marks  one  of  the  most  distinct  and  revolutionary  advances  ever  made  in  the  metal- 
lurgy of  iron  and  steel.  The  composition  of  these  steels  varies  greatly:  they  may 
contain  from  0.25  to  one  per  cent  carbon,  generally  not  over  0.75  per  cent;  from  5  to 
25  per  cent  of  tungsten,  generally  between  10  and  20  per  cent;  from  2  to  10  per  cent 
of  chromium,  generally  between  2  and  8  per  cent,  and  seldom  over  0.40  per  cent  of 
manganese.  Vanadium  varying  in  amount  between  0.25  and  one  per  cent  is  now 
generally  added.1  In  some  types  tungsten  is  replaced  in  part  or  wholly  by  molybde- 
num; in  others  a  small  amount  of  molybdenum  is  present  in  addition  to  the  tungsten 
and  chromium.  Properly  treated  high  speed  still  generally  exhibits  a  polyhedral 


Fig.  334.  —  High  speed  steel.  Quenched  at  1280  deg.  C. 
Magnified  1500  diameters.  Tungsten  17.87  per  cent. 
Chromium  3.46  per  cent.  Vanadium  0.81  per  cent. 
Carbon  0.65  per  cent.  (J.  V.  Emmons.) 


(austenitic?)  structure  (Fig.  333)  quite  free  from  carbide  particles.  In  the  presence 
of  large  amounts  of  tungsten  and  chromium,  however,  some  carbide  remains  undis- 
solved  even  after  rapid  cooling  from  a  temperature  close  to  the  melting-point  of  the 
metal  (Fig.  334) .  The  presence  of  a  moderate  amount  of  free  carbide  after  the  high 
heat  treatment  is  not  considered  detrimental;  on  the  contrary  it  appears  to  increase 
the  cutting  properties  of  the  metal. 

In  the  annealed  condition  the  steel  contains  carbide  particles  which  are  probably 
the  more  numerous  the  larger  the  percentage  of  tungsten  and  chromium  present 
(Figs.  335  and  336). 

1  According  to  Edwards  0.3  per  cent  vanadium  allows  a  cutting  speed  10  per  cent  greater,  that 
is,  permits  the  removal  of  10  per  cent  more  metal  in  the  same  time  while  with  0.20  per  cent  vanadium, 
20  per  cent  more  metal  may  be  removed.  The  time  between  regrinding  is  also  increased  by  vanadium. 


356 


CHAPTER   XXI  — ALLOY   STEELS 


The  inventors  of  high  speed  steels  recommended  the  following  treatments  as 
yielding  the  best  results:  (1)  heating  the  tool  slowly  to  about  815  deg.  C.,  then  quickly 
until  its  extreme  edge  showed  indications  of  melting,  (2)  cooling  the  tool  quickly  to 
below  860  deg.  and  then  quickly  or  slowly  to  atmospheric  temperature,  and  (3)  re- 
heating the  tool  to  about  640  deg.  for  five  minutes  (in  a  lead  bath)  followed  by  cool- 
ing in  air.  The  author  believes,  however,  that  tools  of  high  speed  steel  are  now  gen- 
erally heated  to  near  their  melting-point  followed  by  cooling  freely  in  air,  in  an  air 
blast,  or  in  oil,  a  second  treatment  being  rarely  applied. 

While  the  theories  and  hypothesis  advanced  to  account  for  the  remarkable  prop- 
erties of  high  speed  steels  differ  greatly  and  while  the  true  constitution  of  these 


Fig.  335.  —  High  speed  steel.  Typical 
structure  after  annealing.  Magnified 
150  diameters.  (Edwards.) 


Fig.  336.  —  High  speed  steel.  Annealed  at  780  deg.  C. 
Magnified  1500  diameters.  Tungsten  18.36  per  cent. 
Chromium  4.05  per  cent.  Vanadium  1.04  per  cent. 
Carbon  0.59  per  pent.  (J.  V.  Emmons.) 


steels  is  still  to  be  discovered  it  seems  certain  that  in  order  to  produce  what  may  be 
termed  "high  speed  hardness"  (1)  a  certain  condition  must  be  created  by  heating  to 
a  very  high  temperature  and  (2)  that  condition  must  be  retained  by  quick  cooling, 
since  both  high  heating  and  quick  cooling  are  essential  to  the  production  of  high 
speed  hardness.  It  has  been  argued  that  at  a  high  temperature  carbides  are  formed 
of  great  hardness  and  stability  which  upon  being  retained  by  quick  cooling  impart 
high  speed  properties  to  the  steel,  but  it  is  now  more  generally  held  that  the  high 
temperature  treatment  results  in  the  matrix  of  the  steel  absorbing  all  or  nearly  all 
the  carbides,  thereby  acquiring  great  hardness  which  is  retained  if  the  carbides  are 
prevented,  through  quick  cooling,  from  again  separating.  This  theory  is  better  sus- 
tained by  microscopical  and  other  experimental  evidences. 

From  the  structure  of  treated  high  speed  steel  which  is  often  polyhedral  we  nat- 
urally infer  that  the  metal  is  in  an  austenitic  condition.    Other  considerations,  how- 


CHAPTER  XXI  — ALLOY   STEELS  357 

ever,  lead  us  also  to  believe  that  it  is  partly  martensitic.  We  may  consider  its  condi- 
tion as  martensito-austenitic.  Purely  austenitic  steels  like  some  manganese  and 
nickel  steels  possess  but  little  if  any  high  speed  hardness.  To  be  sure  it  may  be,  as 
some  have  claimed,  that  in  use  the  heat  caused  by  the  friction  of  the  tool  causes  a 
tempering  of  the  austenite  into  martensite,  thereby  increasing  the  hardness  and 
hence  the  efficiency  of  the  tool  rather  than  softening  it  as  would  be  the  case  with 
martensitic  carbon  steel.  The  fact  that  austenitic  manganese  and  nickel  steels  be- 
have differently  might  be  explained  on  the  ground  that  the  austenite  of  which  they 
are  composed  is  less  stable  and  therefore  more  readily  converted"  first  into  martensite 
and  then  into  sorbite  with  accompanying  loss  of  hardness  and  hence  of  cutting  power. 
Tungsten  in  other  words,  and  chromium,  appear  to  create  a  more  stable  condition, 
that  is  one  resisting  tempering  much  more  effectively. 

Clearly  the  fact  that  treated  high  speed  steel  is  probably,  to  a  large  extent  at 
least,  austenitic,  is  not  alone  sufficient,  in  the  light  of  our  present  knowledge,  to 
account  for  its  extraordinary  cutting  properties. 

We  naturally  turn  to  the  heating  and  cooling  curves  of  high  speed  steel  for  addi- 
tional information. 

Mr.  M.  Yatsevitch  of  the  Technical  School  of  Kiev,  Russia,  has  conducted  in 
the  author's  laboratory  an  extensive  investigation  of  the  influence  of  heat  treatment 
upon  the  position  of  the  critical  points  of  high  speed  steel.  Over  100  heating  and 
cooling  curves  were  taken  with  a  Saladin-Le  Chatelier  recording  thermo-electric 
pyrometer.  The  most  significant  results  are  here  briefly  outlined.  The  steel  used, 
kindly  supplied  by  Dr.  J.  A.  Mathews,  had  the  following  compositions:  tungsten 
18.20  per  cent,  chromium  3.00  per  cent,  vanadium  0.95  per  cent,  silicon  0.27  per 
cent,  carbon  O.G2  per  cent,  manganese  0.38  per  cent,  sulphur  less  than  0.025  per  cent, 
and  phosphorus  less  than  0.025  per  cent.  Heating  curves  obtained  on  heating 
this  steel  respectively  to  920,  950,  1000,  and  1050  deg.  C.  are  shown  in  Figure  337 
as  well  as  the  cooling  curves  resulting  from  slow,  furnace  cooling  from  these  tempera- 
tures. 

The  heating  curves  exhibit  two  critical  points  respectively  at  about  775  and  855 
deg.  C.  These  points  were  found  to  be  remarkably  constant.  Occurring  as  they  do 
within  such  narrow  range  of  temperature  (some  80  deg.  C.)  it  might  be  asked  if  in- 
stead of  indicating  two  distinct  transformations  they  do  not  correspond  to  two 
phases  or  stages  of  the  same  transformation. 

The  curves  obtained  on  cooling  slowly  from  920  and  950  deg.  C.  respectively 
indicate  a  sharp  evolution  of  heat  occurring  at  730  to  740  deg.  C.  and  which  we 
naturally  infer  to  correspond  to  the  opposite  phase  of  the  transformation  or  transfor- 
mations occurring  on  heating  as  indicated  by  the  heating  curves.  So  long  as  the 
temperature  reached  on  heating  does  not  exceed  950  deg.  C.  the  curves  obtained  on 
slow  cooling  are  of  the  same  type,  i.e.  they  exhibit  but  one  well-marked  critical  point 
at  730  to  740  deg.  C.  When,  however,  the  maximum  temperature  on  heating,  which 
for  brevity  and  following  Howe,  may  be  called  T"1""  reaches  1000  deg.  C.,  this  ther- 
mal point  is  slightly  lowered  while  a  second  point  occurs  whose  position  although 
shifting  somewhat,  remains  in  the  vicinity  of  400  deg.  C.  (Fig.  337).  As  Tmax  in- 
creases the  upper  point  of  the  curve  obtained  on  slow  cooling  decreases  in  intensity, 
while  the  intensity  of  the  lower  point  increases.  Bearing  in  mind  that  as  Tmax 
increases  we  are  approaching  the  treatment  needed  to  confer  high  speed  hardness 
we  are  justified  in  inferring  that  the  transformations  indicated  by  the  upper  point 


358 


CHAPTER  XXI  — ALLOY   STEELS 


on  cooling  must  be  thoroughly  suppressed  in  order  to  obtain  the  desired  results  and 
that  this  suppression  is  to  be  brought  about  (1)  by  high  heating  and,  as  will  be  shown 
presently,  (2)  by  rapid  cooling. 

The  influence  on  the  cooling  curves  of  rapid  cooling  from  increasingly  high  tem- 
peratures is  clearly  written  in  the  curves  shown  in  Figures  338  and  339.    The  curves 


1050 
IOOO 


300 
200 


IOOO 
950 


Fig.  337.  - 
furnace, 
vitch  in 


—  Heating  and  cooling   curves   of   high   speed   steel.      Slowly  cooled   in 
Time  of  cooling  from   1000  to  200  deg.  C.,  132  minutes.     (M.  Yatse- 
the  author's  laboratory.) 


of  Figure  338  were  obtained  by  cooling  freely  in  air  the  specimens  and  the  tubes  in 
which  they  were  heated. 

Those  of  Figure  339  by  cooling  in  the  same  manner  but  more  rapidly  by  means 
of  a  fan. 

With  the  first  rate  of  cooling  (Fig.  338)  it  will  be  noted  that  (1)  on  cooling  from 
800  deg.  C.  but  one,  sharply  marked  evolution  of  heat  occurs  on  cooling  at  665  deg. 


CHAPTER   XXI  — ALLOY   STEELS 


359 


S.  5 


360 


CHAPTER   XXI  — ALLOY   STEELS 


g  8 

o    6 


8        3 


Mils 

8 


CHAPTER   XXI  — ALLOY    STKKLH  361 

C.,  (2)  on  cooling  from  850,  a  critical  point  occurs  in  the  curve  at  245  cleg.  C.,  and  (3) 
an  increasing  Tmox  up  to  1000  (leg.  O.  the  upper  point  on  cooling  is  somewhat  lifted 
but  greatly  decreases  in  intensity  and  shows  signs  of  being  finally  eliminated,  while 
the  lower  point  is  gradually  raised  to  370  deg.  C.  and  becomes  very  intense.  On  cool- 
ing from  900  (leg.  C.  the  two  points  appear  to  have  nearly  the  same  intensity. 

When  cooling  at  the  more  rapid  rate  indicated  by  the  curves  of  Figure  339  the 
same  occurrences  are  observed  but  still  more  intensified.  Cooling  from  1000  deg.  C. 
for  instance  shows  an  intense  point  at  395  deg.  C.  while  the  upper  transformation 
if  it  still  takes  place  is  much  more  feeble  beside  being  gradual  ami  covering  a  wide 
range  of  temperature  (730  to  4(iO  deg.  C.). 

Summing  up,  it  seems  evident  that  with  increasing  Tmax  and  rate  of  cooling  the 
upper  critical  point  of  the  cooling  curves  is  nearly,  if  not  entirely,  suppressed  while 
the  lower  point  grows  enormously  in  intensity.  The  inference  appears  warranted 
that  the  treatment  imparting  high  speed  hardness  suppresses,  at  least  for  the  major 
part,  a  certain  transformation  occurring  normally  on  slow  cooling  from  moderately 
high  temperatures  at  about  730  deg.  C.,  while  inducing  with  increasing  magnitude  a 
new  transformation  generally  between  350  and  400  deg.  C.  The  conclusion  that  the 
lower  point  is  merely  the  upper  point  depressed  by  high  heating  and  quick  cooling 
would  be  untenable  since  the  cooling  curves  show  conclusively  that  the  upper  point 
is  not  lowered  but  merely  decreases  in  intensity.  As  to  the  nature  of  the  transforma- 
tion apparently  suppressed  and  of  the  transformation  induced  by  the  high  speed 
treatment,  any  positive  statement  would  be  premature.  We  naturally  connect  the 
upper  point  with  the  separation  of  carbide  on  slow  cooling  but  the  cause  of  the  lower 
point  is  still  a  matter  for  further  investigation. 

The  etching  of  high  speed  steel  was  carefully  investigated  in  the  author's  labora- 
tory by  Mr.  M.  Yatsevitch  and  the  following  conclusions  appear  warranted. 

Alcoholic  solution  of  picric  and  nitric  acid  may  be  used  for  etching  high  speed 
steel,  at  times  with  satisfactory  results.  Picric  acid  should  be  in  the  form  of  a  satu- 
rated solution  in  alcohol  and  its  action  may  be  intensified  by  the  addition  of  some  20 
per  cent  of  water.  In  the  absence  of  water  picric  acid  acts  very  slowly,  the  needed 
time  of  etching  moreover  increasing  rapidly  as  the  temperature  from  which  the  steel 
cools  increases  and  as  the  speed  of  cooling  increases,  in  other  words  as  the  conditions 
of  the  industrial  treatment  of  high  speed  steel  are  approached.  Picric  acid  etching 
is  satisfactory  for  steel  in  the  annealed  condition  as  shown  in  Figure  340,  where  the 
bright  particles  of  carbide  are  clearly  revealed  embedded  in  a  dark  matrix.  The 
time  of  etching  was  (5  minutes.  The  same  steel  after  high  heat  treatment  could  not 
be  etched  by  picric  acid  even  after  a  48  hour  immersion. 

Nitric  acid  in  alcohol  (some  10  per  cent  acid)  is  less  satisfactory  for  etching  an- 
nealed specimens  (Fig.  341)  but  much  more  effective  in  revealing  the  structure  of 
heat  treated  specimens  (Fig.  342).  Its  action  while  much  more  rapid  than  that  of 
picric  acid  also  becomes  decidedly  slower  as  the  temperature  reached  on  heating  and 
as  the  speed  of  cooling  increases  and  therefore  as  the  mineralogical  hardness  of  the 
steel  increases.  The  annealed  specimen  shown  in  Figure  341  was  etched  for  2  min- 
utes while  after  high  heat  treatment  (Fig.  342)  an  immersion  of  14  minutes  was 
required.  It  will  be  observed  that  both  picric  and  nitric  acid  etching  leave  the  free 
carbides  bright  while  darkening  the  matrix  and  that  nitric  acid  readily  reveals  the 
polyhedral  structure  of  heat  treated  samples.  It  is  also  apparent  that  the  length  of 
time  needed  to  develop  the  structure  of  high  speed  steel  is  an  indication  of  the  heat 


Fig.  340.  —  Annealed.    Etched  with  picric  acid  six  minutes. 


Fig.  341.  —Annealed.    Etched  with  nitric  acrid  two  minutes. 


Fig.  342.  —  Heated  to  1200  deg.  C.  and  quickly  cooled. 
Etched  with  nitric  acid  fourteen  minutes. 


*^ 


Fig.  343.  —  Annealed.    Etched  with  hydrogen  peroxide  and  Fig.  344.  —  Heated  to  1240  deg.  C.  and  quickly  cooled, 

sodium  hydrate.  Etched  with  hydrogen  peroxide  and  sodium  hydrate. 

High  speed  steel.  Tungsten  18.20  per  cent,  chromium  3  per  cent,  vanadium  0.95  per  cent,  carbon 
0.62  per  cent,  silicon  0.27  per  cent,  manganese  0.38  per  cent.  All  photomicrographs  are  magnified 
980  diameters.  (M.  Yatsevitch  in  the  author's  laboratory.) 


CHAPTER  XXI  — ALLOY   STEELS 


363 


treatment  it  has  received.  Obviously,  increasing  temperature  and  speed  of  cooling 
produce  in  the  matrix  of  high  speed  steel  changes  which  are  not  brought  out  in  the 
etched  structure  of  the  metal  but  which  are  made  apparent  by  a  much  greater  resist- 
ance to  the  action  of  picric  and  nitric  acid  and  also  by  increased  mineralogical  hard- 
ness. 

As  briefly  described  in  Chapter  II,  Mr.  Yatsevitch  has  obtained  interesting  and 
valuable  results  in  using  for  etching  high  speed  steel  a  solution  of  hydrogen  peroxide 
and  sodium  hydrate.  This  reagent  etches  the  particles  of  free_  carbide  dark  while 
leaving  the  matrix  white  and  brilliant  even  after  prolonged  immersion  (Figs.  343 


-- 


A 


Fig.  345.  —  Magnified  86  diameters.  Fig.  346.  —  Magnified  980  diameters. 

High  speed  steel.     Cast.     Etched  with  hydrogen  peroxide  and  sodium  hydrate. 
(M.  Yatsevitch  in  the  author's  laboratory.) 


and  344).  The  etching  time  (about  10  to  15  minutes)  remains  the  same  regardless 
of  the  heat  treatment  applied.  The  method  affords  a  sure  means  of  revealing  the 
existence  of  the  free  carbide  to  the  exclusion  of  everything  else.  It  does  not  reveal 
the  polyhedral  structure  of  the  steel  after  high  heat  treatment,  clearly  because  the 
net  surrounding  the  grains  does  not  indicate  the  presence  around  them  of  membranes 
of  free  carbide. 

The  cast  structure  of  the  high  speed  steel  used  in  these  experiments  is  illustrated 
in  Figures  345  and  346.  It  may  be  safely  inferred  that  the  constituent  colored  dark 
by  the  Yatsevitch  reagent  is  the  carbide  and  that  the  latter  forms  a  component 
therefore  of  the  eutectic  or  eutectoid  evidently  present  in  the  cast  metal. 


CHAPTER  XXII 

CAST   IRON 

Cast  iron  differs  from  steel  in  being  deprived  of  malleability.  This  lack  of  malle- 
ability is  due  to  the  presence  of  a  large  quantity  of  carbon,  generally  between  2.50  and 
4.00  per  cent.  The  carbon  present  in  cast  iron  may  be  (1)  wholly  in  the  graphitic 
condition,  (2)  wholly  in  the  combined  condition,  and  (3)  partly  in  the  graphitic  and 
partly  in  the  combined  condition.  These  three  types  of  cast  iron  should  be  separately 
considered.  The  solubility  of  carbon  in  iron  and  the  factors  influencing  the  forma- 
tion of  graphite  or  of  combined  carbon  should,  however,  first  be  considered. 

Solubility  of  Carbon  in  Iron.  —  Slightly  above  its  melting-point  (1500  deg.  C.) 
pure  iron  does  not  appear  to  be  able  to  dissolve  more  than  some  5  per  cent  carbon. 
As  its  temperature  increases,  however,  its  saturation  point  for  carbon  increase? 
materially.  0.  Ruff  and  0.  Goecke  report  the  following  results: 

TEMPERATURE  PER  CENT 

DEG.  C.  CAKBON  DISSOLVED 

1135 4.30 

1220 4.50 

1305 4.81 

1522 5.4(i 

1623 .  .  5.78 

1823 6.59 

2020 6.95 

2122 7.51 

2169 ....  8.21 

2220 9.60 

2271 8.97 

2320 8.61 

2420 8.09 

2475  ...  7.78 

2626 7.45 

Hanemann  has  reported  the  following  saturation  points. 

TEMPERATURE  PER  CENT 

DEG.  C.  CARBON  DISSOLVED 

1133 4.25 

1350 4.50 

1620 5.40 

1880 6.18 

364 


CHAPTER   XXII  — CAST   IRON 


365 


Iron  saturated  with  carbon  is  reported  to  boil  at  2750  deg.  while  pure  iron  boils 
at  a  much  lower  temperature. 

Ruff's  and  CJoecko's  results  are  shown  graphically  in  Figure  347.     Hanemann's 


26OO 
25OO 
240O 
230O 
22OO 
2IOO 
20  OO 
1  900 


* 

C>) 

Cb 

q 


1800 


1700 


S      I60O 

-tvo 

!P      I5OO 

0) 

-- 


I40O 
I3OO 
I20O 
1100 
IOOO 


IO 


°/0  Carbon 

Fig.  347.  —  Solubility  of  carbon  in  iron. 


results  are  indicated  by  crosses  in  the  same  diagram.  Since  the  carbide  Fe3C,  or 
oementite  contains  6.67  per  cent  carbon,  it  may  be  inferred  from  the  results  tabu- 
lated above,  (1)  that  iron-carbon  alloys  saturated  with  carbon  consist  between  1130 


366  CHAPTER   XXII  —  CAST   IRON 

(the  melting-point  of  the  eutectic  as  later  explained)  and  1823  deg.  of  liquid  solu- 
tions of  iron  and  Fe3C,  the  proportion  of  the  latter  constituent  increasing  with  the 
temperature,  and  (2)  that  at  1823  the  alloy  is  liquid  Fe3C.  From  the  fact  that  the 
amount  of  carbon  dissolved  at  2220  deg.  C.,  namely  9.60  per  cent,  is  the  amount 
called  for  by  the  carbide  Fe2C  it  is  further  inferred  (1)  that  between  1823  and  2220  deg. 
the  alloys  are  liquid  solutions  of  Fe3C  and  Fe2C  the  latter  increasing  and  the  former 
decreasing  with  the  temperature,  and  (2)  that  at  2220  deg.  the  alloy  is  liquid  Fe2C. 
At  higher  temperature  some  of  the  Fe2C  breaks  up  into  iron  and  graphite  (Fe2C 
2Fe  +  C)  the  resulting  graphite  floating  to  the  top  of  the  liquid  bath,  thus  explain- 
ing the  smaller  and  decreasing  proportions  of  dissolved  carbon  at  temperatures  ex- 
ceeding 2220  deg.  The  break  in  the  curve  at  1823  (Fig.  347)  corresponds  to  the 
formation  of  Fe3C,  the  break  at  2220  deg.  to  the  formation  of  Fe2C.  Taking  iron 
with  its  maximum  proportion  of  carbon  namely  at  2220  deg.  when  it  is  capable  of 
dissolving  9.60  per  cent  carbon,  as  previously  stated,  it  then  consists  of  liquid  Fe2C. 
On  cooling  it  is  not  capable  of  retaining  so  much  carbon  some  of  it  being  rejected  as 
graphite  through  the  transformation  of  some  of  the  Fe2C  into  Fe3C  and  graphite 
(3Fe2C  =  2Fe3C  +  C).  This  formation  of  Fe3C  at  the  expense  of  Fe2C  with  rejec- 
tion of  graphite  continues  until  at  1823  deg.  Fe3C  only  is  present  (6.69  per  cent  car- 
bon). On  further  cooling  the  alloy  must  continue  to  reject  carbon,  now  through  the 
transformation  of  some  of  the  Fe3C  into  iron  and  graphite  (Fe3C  =  3Fe  +  C)  a 
transformation  which  continues  until  a  temperature  of  1130  deg.  is  reached  when 
the  alloy  contains  4.30  per  cent  carbon  and  when  further  cooling  now  causes  its 
solidification.  The  possibility  of  the  carbide  Fe3C  solidifying  out  of  the  liquid  solu- 
tion being  later  converted  wholly  or  in  part  into  iron  and  graphite  will  be  considered 
in  the  following  pages. 

Formation  of  Combined  and  Graphitic  Carbon.  —  It  will  be  explained  in  Chapter 
XXVI  that  when  cast  iron  solidifies  the  carbon  probably  remains  in  the  combined 
condition  and  that  the  resulting  carbide,  Fe3C,  is  partly  free  and  partly  in  solid  solu- 
tion in  the  iron.  This  Fe3C,  however,  is  an  unstable  compound  and  when  formed  at 
a  high  temperature  it  is  readily  decomposed  into  graphite  and  iron  according  to  the 
reaction 

Fe3C  =  3Fe  +  C 

hence  the  formation  of  graphitic  carbon  in  cast  iron.  Two  factors  are  conspicuous 
in  promoting  the  formation  of  graphitic  carbon,  (1)  a  slow  rate  of  cooling  through  and 
below  the  solidification  period  and  (2)  the  presence  of  silicon.  The  gray,  i.e.  gra- 
phitic, cast  irons  are  generally  those  which  have  been  cast  in  sand  and  hence  slowly 
cooled  and  which  contain  a  relatively  large  percentage  of  silicon.  It  follows  that 
under  otherwise  identical  conditions  and  compositions  a  large  casting  will  become 
more  graphitic  on  solidifying  than  a  smaller  one  since  it  will  cool  more  slowly;  also 
that  of  two  castings  of  equal  size,  cooled  under  like  conditions  and  of  identical  com- 
position except  as  to  their  silicon  contents,  the  one  richer  in  silicon  will  contain  more 
graphitic  carbon.  The  conditions  most  effective  in  preventing  the  formation  of 
graphitic  carbon  and  in  promoting,  therefore,  the  retention  of  carbon  in  its  combined 
form  are  (1)  quick  rate  of  cooling  through  and  below  the  solidification  range  and  (2) 
the  presence  of  much  sulphur  or  manganese. 

Cast  Iron  Containing  only  Graphitic  Carbon.  —  Cast  irons  containing  a  consider- 
able amount  of  graphitic  carbon  are  known  as  gray  cast  irons  because  of  the  appear- 


CHAPTER  XXII  — CAST   IRON 


367 


ance  of  their  fracture  which  is  grayish  or  blackish  and  coarsely  crystalline.  Cast 
irons  containing  the  whole  of  their  carbon  in  the  graphitic  condition  and  therefore 
free  from  combined  carbon  are  extreme  types  seldom  produced.  Their  structure, 
however,  should  be  considered. 

Proceeding  as  we  did  in  the  case  of  steel  we  shall  first  assume  cast  iron  to  be  a  pure 
alloy  of  iron  and  carbon,  free,  therefore,  from  its  usual  impurities.  If  the  whole  of 
the  carbon  is  in  the  graphitic  condition  it  is  evident  that  cast  iron  can  only  contain 
the  two  constituents  graphite  and  iron  or  ferrite.  We  may  therefore  anticipate  its 
structure.  It  will,  however,  be  interesting  to  study  the  mode -of -occurrence  of  the 


Fig.  348.  —  Gray  cast  iron  free  from  combined  carbon.  Magnified 
100  diameters.  Not  etched.  (F.  C.  Langenberg  in  the  author's 
laboratory.) 

graphitic  carbon.  The  structure  of  cast  iron  practically  free  from  combined  carbon 
is  illustrated  both  before  and  after  etching  in  Figures  348  and  349.  The  metal  will 
be  seen  to  consist  of  an  iron  or  ferrite  matrix  in  which  are  embedded  many  irregular 
and  generally  elongated  and  curved  plates  of  graphite.  These  graphite  plates  break 
up  so  effectively  the  continuity  of  the  metallic  mass  as  to  completely  destroy  the 
ductility  and  malleability  of  a  substance  (ferrite)  by  nature  very  ductile  and  malleable. 
The  brittleness  of  high  graphitic  cast  iron  is  not  due  so  much  to  the  brittleness  of  the 
graphite  it  contains  nor  even  to  its  large  proportion  of  graphite  as  to  the  thorough 
manner  in  which  the  continuity  of  its  otherwise  ductile  matrix  is  destroyed  by  the 
shape  and  distribution  of  the  graphite  particles. 

It  will  be  seen  in  another  chapter  that  when  the  graphite  occurs  in  small  rounded 
particles  as  it  does  in  malleable  cast  iron  the  ferrite  matrix  may  retain  considerable 
ductility  and  malleability. 


368 


CHAPTER   XXII  — CAST   IRON 


The  ferrite  matrix  of  this  highly  graphitic  cast  iron  (Fig.  349)  will  be  seen  to  he- 
made  up  of  the  polyhedral  crystalline  grains  characteristic  of  carbonless  iron,  the  fer- 
rite of  cast  iron  being  similar  in  this  and  other  respects  to  the  ferrite  of  wrought  iron 
and  of  hypo-eutectoid  steel.  In  impure  cast  iron  it  undoubtedly  holds  in  solution 
silicon  and  possibly,  to  some  extent,  other  impurities. 

Highly  graphitic  cast  iron  is  brittle  and  deprived  of  ductility  and  malleability 
because  of  the  presence  of  numerous  plates  of  graphitic  carbon;  it  is  weak  because  of 
the  presence  of  graphite  plates  and  because  of  the  relative  weakness  of  its  matrix;  it 
is  soft  and  therefore  easily  machined  because  of  the  softness  both  of  its  matrix  and 
of  the  graphite  it  contains;  it  expands  on  solidifying  because  of  the  formation,  with 


Fig.  349.  —  Same  metal  as  in  Figure  348.  Magnified  100  diameters. 
Etched.     (F.  C.  Langenberg  in  the  author's  laboratory.) 


increase  of  bulk,  during  solidification  of  a  large  amount  of  graphitic  carbon.  It 
should  be  noted  that  because  of  its  low  specific  gravity  graphite  will  occupy  a  rela- 
tively large  proportion  of  the  bulk  of  the  metal.  Cast  iron,  for  instance,  containing 
by  weight  3  per  cent  of  graphite  contains  by  volume  some  12  per  cent  of  that  element- 
As  might  be  expected  the  rate  of  solidification  and  further  cooling  has  some  in- 
fluence both  upon  the  shape  and  size  of  the  graphite  particles  as  well  as  upon  the 
size  of  the  ferrite  grains,  and  therefore  upon  the  physical  properties  of  the  metal, 
very  slow  solidification  promoting  the  formation  of  large  graphite  plates  and  of  large 
ferrite  grains.  Were  it  possible  to  cause  the  graphite  in  cast  iron  to  occur  in  small 
rounded  particles  instead  of  sharp,  curved  plates,  its  ductility  and  strength  would 
undoubtedly  be  greatly  increased. 

The  diagram  of  Figure  350  shows  graphically  the  structural  composition  of  iron- 


CHAPTER  XXII  —  CAST   IROX 


369 


carbon  alloys  in  which  the  whole  of  the  carbon  occurs  as  graphite.  Only  those  alloys, 
however,  containing  from  3  to  4.5  per  cent  carbon  can  be  produced.  Indeed  in  the 
absence  of  silicon  even  these  are  quite  unobtainable.  With  less  than  3  per  cent  car- 
bon it  is  well  nigh  impossible  to  prevent  the  retention  of  some  combined  carbon, 
while  with  less  than  2  or  at  least  with  less  than  1.50  per  cent  carbon  the  whole  of  the 
carbon  is  likely  to  be  in  the  combined  condition.  The  diagram,  therefore,  is  only  a 
theoretical  one.  It  has,  nevertheless,  its  interest  for  it  will  be  shown  in  another  chap- 


/OO 


Cast /ron 
free  from 

".ombinectcarkx. 


Graph  //e 
by  weighi". 


Percent    Carbon 

Fig.  350.  —  Structural  composition  diagram  of  iron-carbon  alloys  free  from  combined  carbon. 


ter  to  represent  the  stable  and  final  equilibrium  of  the  iron-carbon  system.  The 
percentage  of  graphite  by  volume  has  also  been  indicated. 

Cast  Iron  Containing  only  Combined  Carbon.  —  Cast  iron  containing  only  com- 
bined carbon  and  free,  therefore,  from  graphitic  carbon  is  called  "white"  cast  iron 
from  the  aspect  of  its  fracture  which  is  white,  brilliant,  and  highly  metallic.  The 
absence  of  graphitic  carbon  is  generally  due  (1)  to  the  presence  of  much  manganese 
and  sulphur  and  of  little  silicon,  (2)  to  quick  cooling  through  and  below  the  solidifica- 
tion period,  or  (3)  to  both  low  silicon,  high  manganese  and  sulphur,  and  quick  solidi- 
fication. Cast  iron,  for  instance,  may  contain  so  much  sulphur  and  manganese  and 
so  little  silicon  as  to  be  white  even  after  slow  solidification  or  it  may  solidify  so  quickly 
as  to  be  white  even  in  the  presence  of  much  silicon  and  little  manganese  and  sulphur. 

A  familiar  instance  of  the  marked  influence  of  the  rate  of  cooling  is  afforded  by 


370 


CHAPTER  XXII  — CAST   IRON 


the  casting  in  the  metal  molds  of  casting  machines  of  cast  iron  which  if  cast  in  sand 
would  have  been  gray,  whereas  it  is  now  white.  Small  castings  since  they  cool  more 
quickly  become  white  more  readily  than  larger  ones. 

In  the  absence  of  graphitic  carbon  the  structure  of  cast  iron  should  resemble  the 
structure  of  a  very  high  carbon  steel,  i.e.  it  should  consist  after  slow  cooling  of  pearlite 
and  of  a  large  amount  of  free  cementite.  This  is  found  to  be  the  case  as  shown  in 
Figure  351  in  which  is  illustrated  the  structure  of  white  iron  containing  about  3  per 
cent  of  combined  carbon.  Theoretically  this  alloy  should  contain  nearly  63  per  cent 
of  pearlite  and  37  per  cent  of  free  cementite.  The  dark  constituent  in  the  photograph 
is  pearlite,  the  light  one,  visibly  in  relief,  cementite.  The  structure  of  white  cast 
iron  is  also  shown  in  Figure  352  under  high  magnification,  the  laminations  of  pearlite 
being  clearly  seen.  The  structure  of  white  cast  iron  is  further  illustrated  in  Figures 


Fig.  351.  —  White  cast  iron.     Magnified  56 
diameters. 


Fig.  352.  —  White   east    iron.      Magnified 
500  diameters.     (Wust.) 


353  and  354  after  Guillet.  These  photomicrographs  are  reproduced  here  because  they 
afford  an  interesting  example  of  the  action  of  sodium  picrate  (Chapter  VIII)  in 
coloring  free  cementite  while  leaving  pearlite  uncolored. 

The  structural  composition  of  white  cast  iron  is  to  be  calculated  like  the  composi- 
tion of  any  hyper-eutectoid  steel  of  known  carbon  content  as  explained  in  Chapter 
VIII,  the  following  relation  existing  between  the  percentage  of  pearlite  and  that  of 
carbon  in  the  iron,  on  the  assumption  that  pearlite  contains  0.834  per  cent  carbon: 


P  = 


800  -  120  C 


the  balance  of  the  metal  being  of  course  free  cementite.  While  structurally  it  re- 
sembles high  carbon  steel,  white  cast  iron  is  deprived  of  malleability  being  indeed 
very  brittle  and  very  hard.  This  brittleness  and  hardness  are  due  to  the  very  large 
proportion  of  free  cementite  present  which  itself  is  very  hard  and  brittle. 


CHAPTER   XXII  — CAST   IRON 


371 


It  will  be  evident  that,  starting  from  carbonless  iron,  as  the  carbon  increases  at 
first  low  carbon  steel  is  produced  and  then  in  succession  medium  high  carbon  steel, 
high  carbon  steel,  and  finally  white  cast  iron,  each  metal  passing  gradually  into  the 
next  without  any  sharp  line  of  demarcation  between  them.  It  is  logical  to  base  the 
distinction  between  high  carbon  steel  and  white  cast  iron  upon  the  malleability  of 
the  former  and  the  non-malleability  of  the  latter  and  this  is  altogether  a  question  of 
carbon  content.  The  dividing  line  may  be  drawn  somewhat  arbitrarily  at  2  per  cent 
carbon.  As  a  matter  of  fact  steels  are  very  seldom  manufactured  containing  more 
than  1.75  per  cent  carbon  while  white  cast  iron  rarely  contaiusJess  than  2.25  per 
cent  carbon.  Between  the  steel  series,  therefore,  and  the  white  cast-iron  series  there 
is  a  natural  gap,  the  existence  of  which  generally  removes  any  doubt  as  to  the  nature 
of  the  metal  under  examination. 

Again  if  the  process  of  manufacture  be  known  there  need  be  no  doubt  as  to  the 
classification  of  any  highly  carburized  iron  alloy:  if  made  in  the  blast  furnace  from 


S&  sfc-T^* 
^v«**»  i^v^c^ 

1  v+'to<ffite&  tv 
&/*'W'V&h±  * 

-Mm* 


Fig.  353.  —  White  cast  iron.  Magnified 
200  diameters.  Etched  with  picric  acid. 
(Guillet.) 


Fig.  354.  — •  Same  metal  as  in  Fig.  353.  Mag- 
nified 200  diameters.  Etched  with  sodium 
picrate.  (Guillet.) 


the  reduction  of  iron  ore,  it  is  cast  iron,  while  if  it  is  the  product  of  refining  cast  iron 
(by  the  Bessemer  or  the  open  hearth  processes),  or  of  the  remelting  under  oxidizing 
conditions  of  iron  or  steel  scrap  with  or  without  cast  iron  (open  hearth  process),  or  of 
the  carburizing  of  wrought  iron  (cementation  process),  or  of  the  carburizing  and 
melting  of  wrought  iron  (crucible  process),  it  is  steel. 

The  diagram  of  Figure  355  indicates  graphically  the  structural  composition  both 
proximate  and  ultimate  of  any  iron-carbon  alloy  containing  from  0  to  6.67  per  cent 
combined  carbon,  that  is,  from  100  per  cent  ferrite  to  100  per  cent  cementite  assum- 
ing as  it  has  been  done  before  that  pearlite  contains  0.834  per  cent  carbon. 

It  will  be  noted  that  two  sources  of  ferrite  are  indicated  in  the  diagram,  namely, 
(1)  pearlite  (eutectoid)  ferrite  and  (2)  pro-eutectoid  or  free  ferrite,  the  sum  of  both 
being  known  as  total  ferrite,  while  four  sources  of  cementite  are  to  be  considered, 
namely,  (1)  pearlite  (eutectoid)  cementite,  (2)  pro-eutectoid  cementite,  (3)  eutectic 
cementite,  and  (4)  pro-eutectic  cementite,  the  sum  of  all  four  being  known  as  total 


372 


CHAPTER   XXII  — CAST    IKON 


Fig.  355.  —  Structural  composition  diagram  of  iron-carbon  alloys  free  from  graphitic  carbon. 


CHAPTER   XXII— CAST    IRON'  373 

cement ite  and  that  of  (2),  (3),  and  (4)  as  free  cementite.  Let  us  recall  the  meaning 
of  these  terms : 

Pearlite  or  eutectoid  ferrite  is  the  ferrite  included  in  pearlite. 

Free  or  pro-eutectoid  ferrite  is  the  ferrite  liberated  as  hypo-eutectoid  steel  cools 
slowly  from  its  upper  critical  point  (Ar3  or  Ar3.2)  to  its  lower  point  (Art) . 

Pearlite  or  eutectoid  cementite  is  the  cementite  included  in  pearlite. 

Pro-eutectoid  cementite  is  the  cementite  that  is  liberated  as  hyper-eutectoid  metal 
cools  slowly  from  its  upper  critical  point  (Arcm)  to  its  lower  point  _(Ar3.2.i) . 

Eutectic  cementite  is  the  cementite  included  in  the  austenite-cementite  eutectic 
which  forms  at  the  end  of  the  solidification  of  alloys  containing  more  than  some  1.70 
per  cent  carbon  as  explained  in  Chapter  XXVI. 

Pro-eutectic  cementite  is  the  cementite  which  forms  between  the  beginning  and 
the  end  of  the  solidification  of  alloys  containing  more  than  4.3  per  cent  carbon  as 
explained  in  Chapter  XXVI.  Howe  calls  it  "primary"  cementite.1 

The  portions  of  the  diagram  corresponding  respectively  to  the  steel  series  and  to 
the  white  cast-iron  series  have  also  been  indicated  leaving  two  groups  of  alloys  un- 
represented by  industrial  products,  namely,  those  containing  between  1.70  per  cent 
and  2.25  per  cent  carbon  and  those  containing  more  than  5  per  cent  of  carbon.2  The 
steel  portion  of  this  diagram  has  been  used  in  Chapter  VIII. 

Cast  Iron  Containing  both  Combined  and  Graphitic  Carbon.  —  Cast-iron  castings 
nearly  always  contain  both  combined  and  graphitic  carbon.  In  the  majority  of  cases 
they  contain  from  0.25  to  1.50  per  cent  of  combined  carbon,  the  balance  of  that  ele- 
ment being  in  the  graphitic  condition.  The  chief  factors  affecting  this  distribution 
of  carbon  between  the  combined  and  the  graphitic  states  have  already  been  alluded 
to;  they  are  (1)  the  rate  of  cooling  during  and  below  solidification  (hence  the  size  of 
the  castings)  and  (2)  the  presence  of  silicon,  manganese,  and  sulphur,  the  first  element 
promoting,  and  the  last  two  opposing,  the  formation  of  graphitic  carbon.  If  it  be 
considered  (1)  that  graphitic  carbon  is  soft,  (2)  that  the  presence  of  much  graphite, 
since  it  implies  little  combined  carbon,  means  the  occurrence  of  soft  ferrite  in  the  cast 
iron,  (3)  that  combined  carbon  (cementite)  is  very  hard,  and  (4)  as  later  explained, 
that  the  presence  of  combined  carbon,  at  least  up  to  a  certain  proportion,  greatly  in- 
creases the  strength  of  cast  iron,  it  will  be  evident  that  the  physical  properties  of  cast 
iron,  especially  its  strength  and  hardness,  will  depend  chiefly  upon  the  proportion  of 
combined  and  graphitic  carbon  it  contains. 

Let  us  first  consider  the  structure  of  cast  iron  containing  a  small  amount,  say  0.25 
per  cent,  of  combined  carbon  and  some  3  per  cent  of  graphitic  carbon  (Fig.  356).  The 
combined  carbon  will  yield  15  times  its  own  weight  of  cementite  (0.25  X  15  =  3.75 

1  Howe  questions  the  actual  existence  of  commercial  hyper-eutectic  white  cast  iron  on  the  ground 
that  pro-eutectio  cementite,  as  later  explained,  is  so  rapidly  converted  into  graphite  that  it  cannot 
be  retained  unless  indeed  by  extremely  quick  cooling  or  in  the  presence  of  much  manganese. 

2  It  should  be  remembered  that  we  are  considering  here  pure  alloys  of  iron  and  carbon.    In  the 
presence  of  much  manganese  (or  chromium)  iron  may  contain  as  much  as  6.50  per  cent  and  even  much 
more  carbon,  while  in  their  absence  cast  iron  very  seldom  contains  more  than  4.5  per  cent  carbon. 
Again  some  unimportant  products  are  offered  for  sale  under  the  name  of  semi-steel  which  may  con- 
tain between  1.75  and  2.50  per  cent  of  total  carbon  entirely  combined,  forming,  therefore,  a  sort  of 
connecting  link  between  the  steel  and  the  cast-iron  series.    They  are  frequently  obtained  by  remelt- 
ing  cast  iron  in  cupola  furnaces  in  the  presence  of  considerable  iron  or  steel  scrap.    Washed  metal 
likewise  may  contain  between  1.75  and  2.50  per  cent  of  total  and  entirely  combined  carbon  but  this 
is  a  semi-finished  product  resulting  from  a  partial  refining  only  of  cast  iron. 


374 


CHAPTER   XXII  — CAST   IRON 


per  cent)  and  the  resulting  cementite  8  times  its  own  weight  of  pearlite  (3.75  X 
8  =  30  per  cent),  or  more  quickly, 

0.25  X  120  =  30  per  cent  pearlite 

exactly  as  in  the  case  of  steel. 

The  cast  iron  under  consideration  will  therefore  contain  30  per  cent  pearlite,  3 
per  cent  of  graphite,  and  the  balance,  67  per  cent,  necessarily  free  ferrite.  Cast  iron 
containing  graphitic  carbon  may  be  considered  as  being  made  up  of  two  distinct 
parts,  namely,  (1)  graphite  and  (2)  a  metallic  matrix  in  which  the  graphite  particles 
are  embedded.  It  will  be  obvious,  moreover,  that  the  metallic  matrix  of  gray  cast 
iron  is  in  reality  a  steel  matrix  since  it  necessarily  consists,  like  steel,  of  an  aggregate 


Fig.  356.  —  Gray  cast  iron.  Hypo-eutectoid  matrix 
(0.25  per  cent  combined  carbon).  Magnified  100 
diameters.  (Boylston.) 


of  ferrite  and  cementite  partly  associated  to  form  pearlite.  In  cast  iron  free  from 
combined  carbon  the  matrix  is  pure  ferrite;  with  a  little  combined  carbon  it  is  of  the 
nature  of  a  low  carbon  steel;  as  the  combined  carbon  increases  the  metallic  matrix  is 
converted  into  steel  of  increasing  carbon  content;  with  less  than  some  0.80  per  cent 
carbon  trie  matrix  resembles  an  hypo-eutectoid  steel;  with  some  0.80  per  cent  carbon 
the  matrix  is  pure  pearlite,  i.e.  eutectoid  steel;  with  more  than  0.80  per  cent  carbon 
some  free  cementite  is  formed,  the  matrix  consisting  of  hyper-eutectoid  steel.  In 
other  words,  as  the  proportion  of  combined  carbon  increases,  structural  changes  take 
place  in  the  metallic  matrix  of  cast  iron  identical  to  those  observed  and  described  in 
the  case  of  steel.  The  structure  of  cast  iron  containing  increasing  proportion  of  com- 
bined carbon  is  illustrated  in  Figures  356  to  358. 

The  above  considerations  justify  us  in  considering  gray  cast  iron  as  steel,  that  is, 
as  an  aggregate  of  ferrite  and  cementite,  the  continuity  of  which  is  destroyed  by  the 
presence  of  numerous  graphite  plates,  the  enormous  difference  in  properties  between 
cast  iron  and  steel  being  due  solely  to  the  presence  of  this  graphitic  carbon  or  rather 


CHAPTER   XXII  — CAST   IRON 


375 


to  the  breaking  of  the  continuity  of  the  mass  which  it  implies.  To  illustrate  further, 
if  it  were  possible  to  remove  bodily  from  a  chunk  of  cast  iron  every  particle  of  graphite, 
its  strength  and  ductility  would  not  probably  be  greatly  increased  because  its  con- 
tinuity would  still  be  effectively  destroyed  by  the  empty  spaces  once  occupied  by 
graphite.  If  not  too  high  in  carbon,  however,  it  would  now  be  welclable  and  could  be 
forged  into  a  small  piece  of  steel,  or  it  could  be  remelted  and  cast  into  a  sound  steel 
casting. 

Mottled  Cast  Iron.  —  Cast  irons  are  sometimes  produced  that  are  partly  gray  and 
partly  white,  that  is,  made  up  of  particles  containing  graphitic  carbon  and  of  particles 


Fig.  357.  —  Gray  cast  iron.    Eutectoid  matrix.    Magni- 
fied 500  diameters.     (Boylston.) 


Fig.  358.  —  Gray  cast  iron.    Hyper-eutec- 
toid  matrix.     Magnified  500  diameters. 

(Wiist.) 


free  from  graphite.    Their  structure  is  well  shown  in  Figure  359.    They  are  called 
"mottled"  because  of  the  appearance  of  their  fracture. 

Structural  Composition  of  Cast  Iron.  —  The  structural  composition  of  any  cast 
iron  of  known  percentages  of  graphitic  and  combined  carbon  and  considered  as  a 
pure  alloy  of  iron  and  carbon  can  readily  be  calculated  by  following  the  method 
employed  in  the  case  of  steel  and  assuming  pearlite  to  contain  0.834  per  cent  carbon, 
that  is,  exactly  one  part  by  weight  of  cementite  and  7  parts  of  ferrite.  It  s'hould  be 
noted,  however,  that  in  the  presence  of  graphite  it  requires  a  smaller  proportion  of 
carbon  to  convert  the  whole  matrix  into  pearlite  since  there  is  less  iron  to  be  so 
converted.  To  make  the  matter  clear  in  order  that  cast  iron  may  be  free  from  both 
excess  ferrite  and  excess  cementite  it  must  contain  ferrite  and  cementite  in  the  exact 
proportion  of  one  part  of  cementite  to  7  of  ferrite.  If  the  cast  iron  contains  G  per 
cent  of  graphite  and  C  per  cent  of  combined  carbon  forming  15C  per  cent  of  cem- 
entite the  balance  of  the  metal,  100  —  G  —  15C,  will  be  ferrite.  Consequently  if 

100  -  G  -  15C  =  7  X  15C  =  105C 
or  100  -  G  -  120C  =  0 


376 


CHAPTER   XXII  — CAST   IRON 


that  is,  if  the  proportion  of  ferrite  equals  7  times  that  of  cementite  the  matrix 
will  contain  pearlite  only;  if  100  -  G  -  120C  is  greater  than  0  the  matrix  will  be  hypo- 
eutectoid;  if  100  -  G  -120C  is  smaller  than  0  the  matrix  will  be  hyper-eutectoid. 
In  the  presence  of  3  per  cent  graphite,  for  instance,  the  following  relation  will  indicate 
the  needed  percentage  of  combined  carbon  to  make  the  matrix  entirely  pearlitic : 

100  -  3  -  120C  =  0 

which  gives  for  C  very  nearly  0.80  per  cent. 

It  will  be  sufficiently  accurate  to  assume  in  every  case  that  gray  cast  iron  contain- 
ing less  than  0.80  per  cent  combined  carbon  has  an  hypo-eutectoid  matrix  while  cast 
iron  containing  more  combined  carbon  has  an  hyper-eutectoid  matrix. 


Fig.  359.  —  Mottled  iron.     Magnified  ±00  diameters.     (Boylston.) 


In  calculating  the  structural  composition  of  cast  iron  two  cases  then  should  he 
considered,  (1)  the  cast  iron  contains  less  than  0.80  per  cent  combined  carbon;  ithu* 
an  hypo-eutectoid  matrix  and  (2)  it  contains  more  than  0.80  per  cent  combined  car- 
bon; it  has  an  hyper-eutectoid  matrix.  In  the  first  instance  we  have  the  following 
relations  between  the  percentage  of  graphitic  carbon,  G,  the  percentage  of  combined 
carbon,  C,  the  percentage  of  ferrite,  F,  and  the  percentage  of  pearlite,  P, 

(1)  F  +  P  +  G  =  100 

(2)  P  =  8  X  15C  =  120C 

If  C  =  0.50  per  cent,  for  instance,  and  G  =  3  per  cent,  the  iron  would  contain  60 
per  cent  pearlite,  37  per  cent  ferrite,  and  3  per  cent  graphite. 

If  the  iron  has  an  hyper-eutectoid  matrix,  that  is,  if  it  contains  more  than  0.80 


CHAPTKR   XXII  —  CAST    IKON 


377 


per  cent  of  combined  carbon,  we  can  write  the  following  equations,  Cm  representing 
the  percentage  of  free  cementite, 

(1)  P  +  Cm  +  G  =  100 

(2)  P  =  ?F  =  -2-  (100  -  G  -  15C) 

the  second  equation  expressing  the  fact  that  the  totality  of  the  ferrite  (100  —  G  —  15C) 
is  included  in  the  pearlite  and  that  the  percentage  of  pearlite  is  equal  to  |  times  that 
of  ferrite.  If  cast  iron  contains  2  per  cent  of  graphite  and  1.25j>er  cent  of  combined 
carbon,  for  instance,  the  foregoing  equations  indicate  the  following  structural  com- 
position: pearlite  90.60  per  cent,  free  cementite  7.40  per  cent,  and  graphite  2  per  cent. 


Ca-sf  /ron 

w/fh  Hypo- 

matr/x 


Cast  /ron 
with  Hyper-eufectoid 

matrtx 


Graphite 


•*>; 


•fx 

i 

o 


Pearlite  (ei/fecfa/e/J-ferr/te. 


Fre 


Combined   C   °fo 
Graphite      C   -ft. 


o. 


O-5"O 
&oo 


/.OO 
Z.5o 


/JO 
2..OO 


2.0O 
/.SO 


2.  SO 
/.OO. 


30O 

OSO 


3.3O 
O 


Fig.  360.  —  •  Structural  composition  diagram  of  iron-carbon  alloys  containing  a  constant  propor- 
tion of  total  carbon  (3.50  per  cent),  but  varying  percentages  of  combined  carbon  (from  0  to  3.50 
per  cent). 


The  structural,  graphical  diagram  of  cast  iron  containing  both  combined  and 
graphitic  carbon  has  been  constructed  in  Figure  360  in  accordance  with  the  scheme 
followed  in  these  chapters.  It  is  assumed  in  this  diagram  that  the  total  carbon  re- 
mains constant  at  3.50  per  cent  and  that  the  amount  of  combined  carbon  increases 
from  0  to  3.50  per  cent,  in  this  way  including  the  two  extreme  cases  corresponding 
respectively  to  absence  of  combined  carbon  and  of  graphitic  carbon.  If  this  diagram 
be  compared  with  that  of  the  structural  composition  of  steel,  Chapter  VIII,  the 
steel  nature  of  the  metallic  matrix  of  cast  iron  will  be  apparent.  It  will  be  noted  that 
in  the  present  diagram  when  the  proportion  of  combined  carbon  exceeds  1.7  per  cent 
there  are  two  sources  of  free  cementite  indicated,  namely,  pro-eutectoid  cementite 


378  CHAPTER  XXII  — CAST  IRON 

and  eutectic  cementite;  the  origin  of  the  latter  will  be  made  clear  in  Chapter  XXVI. 
Both  of  these  cementites  constitute  the  free  cementite  present  in  cast  iron,  containing 
more  than  1.7  per  cent  of  combined  carbon;  while  formed,  as  later  explained,  at  dif- 
ferent periods  of  the  cooling,  they  appear  to  coagulate  together  and  cannot  be  dis- 
tinguished from  each  other  under  the  microscope. 

Physical  Properties  of  Cast  Iron  vs.  its  Structural  Composition.  —  The  physical 
properties  of  cast  iron  must  necessarily  depend  to  a  very  great  extent  upon  the  prop- 
erties of  its  steel  matrix  from  which  it  follows  that  its  hardness  and  strength  will 
increase  with  increasing  combined  carbon,  the  hardness  indefinitely,  the  strength  up 
to  the  eutectoid  carbon  ratio.  It  is  evident,  therefore,  that  cast  iron  of  maximum 
strength  (1)  should  have  a  steel  matrix  of  maximum  strength,  i.e.  should  contain 
in  the  vicinity  of  0.80  per  cent  combined  carbon  and  (2)  should  contain  as  little 
graphitic  carbon  as  possible  since  every  graphite  particle  is  a  source  of  weakness; 
in  other  words,  the  nearer  cast  iron  approaches  a  steel  of  maximum  strength  the  greater 
will  be  its  strength.  After  having  secured  the  desired  amount  of  combined  carbon  to 
give  strength  it  is  evident  that  a  reduction  of  the  graphitic  carbon  must  mean  a  cor- 
responding reduction  of  the  total  carbon  in  cast  iron.  In  ordinary  cupola  practise  for 
the  production  of  cast-iron  castings,  however,  which  consists  in  remelting  pig  iron  of 
suitable  composition,  the  proportion  of  total  carbon  is  difficult  to  control,  being  nec- 
essarily between  3  and  4  per  cent  and  we  must  depend  to  produce  strength  almost 
altogether  upon  the  retention  in  the  combined  condition  of  a  suitable  proportion  of 
carbon.  The  total  carbon  may  be  decreased,  however,  by  the  use  of  iron  and  steel 
scrap  as  part  of  the  burden  of  the  cupola  resulting  in  increased  strength,  for  same 
percentage  of  combined  carbon,  or  by  remelting  in  a  so-called  "air  furnace,"  i.e. 
under  oxidizing  conditions  when  part  of  the  carbon  is  burnt  out.  These  low  total 
carbon,  and  therefore  tenacious,  cast-iron  castings  are  sometimes  offered  for  sale 
under  the  name  of  semi-steels,  a  practise  somewhat  misleading  for  they  are  not  steel 
in  any  sense  of  the  word  since  they  are  not  malleable,  have  very  little  ductility,  and 
generally  contain  a  considerable  amount  of  graphitic  carbon. 

If  soft  cast-iron  castings  are  desired  so  that  they  may  be  easily  machined  they 
should  contain  as  little  combined  carbon  as  possible.  In  the  presence  of  but  little 
combined  carbon,  however,  the  iron  will  not  be  very  tenacious,  strength  and  softness 
being  antagonistic.  If  the  castings  are  to  be  hard  they  should  contain  much  com- 
bined carbon  and,  therefore,  little  graphite.  In  extreme  cases  they  will  be  free  from 
graphite,  when  their  hardness  will  be  very  great,  but  they  will  then  also  be  very  brittle. 
In  the  majority  of  cases  castings  are  wanted  soft  enough  to  be  easily  machined  and  at 
the  same  time  of  fair  strength.  This  combination  of  properties  is  evidently  to  be 
obtained  by  producing  a  matrix  corresponding  to  a  medium  high  carbon  steel,  i.e.  by 
causing  the  cast  iron  to  retain  some  0.30  to  0.60  per  cent  of  combined  carbon. 

The  percentage  of  combined  carbon  in  cast  iron  upon  which  its  physical  properties 
primarily  depend,  can  be  ascertained  more  quickly  and  readily  by  microscopical  ex- 
amination than  by  chemical  analysis  and  quite  as  accurately. 

Chilled  Cast-iron  Castings.  —  It  is  sometimes  desired  to  produce  cast-iron  cast- 
ings very  hard  near  their  outside  but  soft  and  relatively  tough  near  their  center.  This 
may  be  done  by  so  regulating  the  composition  and  solidification  as  to  prevent  the 
formation  of  graphite  in  those  portions  that  should  be  hard  while  allowing  it  to  form 
in  the  portions  that  should  be  soft.  The  means  generally  employed  consist  in  using 
iron  plates  for  those  parts  of  the  molds  corresponding  to  the  parts  of  the  castings  that 


CHAPTER   XXII  — CAST    IRON 


379 


are  to  be  hard  and  sand  for  the  other  parts,  the  quicker  solidification  and  further  cool- 
ing of  the  metal  coming  in  contact  with  the  iron  plates  causing  the  retention  of  much 
combined  carbon.  The  resulting  castings  are  known  as  "chilled"  castings.  Impor- 
tant instances  of  the  application  of  this  method  are  to  be  found  in  the  manufacture 
of  chilled  cast-iron  wheels  and  of  chilled  rolls.  It  will  be  evident  that  the  presence 
of  sulphur  and  manganese  in  the  cast  iron  should  promote  the  retention  of  combined 
carbon  on  quick  cooling  while  the  presence  of  silicon  and  of  large  percentages  of  total 
carbon  should  hinder  it.  The  chemical  composition  of  cast  iron  to  be  converted  into 
chilled  castings  should  therefore  be  carefully  regulated.  Microscopical  examination 
should  prove  of  much  value  in  examining  the  depth  and  quality  of  "chills." 

Cast  Iron  of  Eutectic  Composition.  —  The  alloy  of  iron  and  carbon  containing 
4.30  per  cent  carbon  has  the  lowest  melting-point  (1130  deg.  C.)  of  all  alloys  of  that 


o  i  2 

PER  CENT  SILICON 

Fig.  361.  —  Influence  of  silicon-content  on  the  percentage  of  carbon 
in  the  eutectic  alloy. 


series  and  is  consequently  known  as  eutectic  alloy.  The  nature  of  eutectic  alloys 
will  be  made  clear  in  Chapter  XXV.  Those  alloys  solidify  at  a  constant  temper- 
ature whereas  the  solidification  of  alloys  containing  more  or  less  carbon  than  the  eu- 
tectic ratio  covers  a  range  of  temperature  which  increases  in  width  as  the  compo- 
sition of  the  alloy  is  farther  removed  from  that  of  the  eutectic  as  explained  in  Chapter 
XXVI.  In  other  words  eutectic  alloys  pass  quickly  from  the  liquid  to  the  solid  state 
while  hypo-  and  hyper-eutectic  alloys  pass  through  a  pasty  or  semi-fluid  period 
which  may  be  of  considerable  duration,  a  condition,  as  later  explained,  which  must  be 
favorable  to  the  formation  of  large  graphite  particles. 

Eutectic  Cast  Iron  vs.  Impurities.  —  While  as  stated  in  the  preceding  paragraph 
the  eutectic  point  for  pure  alloys  of  iron  and  carbon  corresponds  to  4.30  per  cent 
carbon  it  is  quite  certain  that  some  at  least  of  the  impurities  always  present  in  commer- 
cial cast  iron  affect  in  a  marked  degree  the  percentage  of  carbon  needed  to  produce 
the  eutectic  alloy,  i.e.  the  alloy  of  lowest  melting-point  generally  reducing  the  needed 
amount  of  carbon.  From  the  work  of  Wust  and  Petersen,  for  instance,  we  are  led  to 
infer  that  the  presence  of  1  per  cent  of  silicon  lowers  that  carbon  some  0.30  per  cent. 


380  CHAPTER  XXII  — CAST  IRON 

These  authors'  results  may  profitably  be  represented  graphically  as  shown  in  Figure 
361.  The  meaning  of  the  diagram  is  obvious.  The  line  AB  divides  cast  iron  into 
hyper-  and  hypo-eutectic  metal  according  to  its  percentages  of  carbon  and  silicon. 
With  4  per  cent  of  carbon  and  2  per  cent  of  silicon,  for  instance,  the  cast  iron  is  hyper- 
eutectic,  since  its  composition  is  represented  by  the  point  M  in  the  hyper-eutectic 
range,  while  with  the  same  amount  of  carbon  but  in  the  presence  of  only  0.5  per  cent 
of  silicon,  the  metal  is  hypo-eutectic,  since  its  composition  is  represented  by  the  point 
N  in  the  hypo-eutectic  range;  with  3  per  cent  silicon  some  3.50  per  cent  carbon  suffices 
to  produce  the  eutectic  alloy,  etc. 

The  Strength  of  Cast  Iron  vs.  the  Size  and  Form  of  the  Graphite  Particles.  — 
Attention  has  already  been  called  to  the  fact  that  the  strength  of  cast  iron  is  not  only 
affected  by  the  amount  of  carbon  present  but  also,  and  in  a  marked  degree,  by  the 
size  and  form  of  the  graphite  particles,  long,  curved  plates  of  that  constituent,  for 
instance,  being  much  more  effective  in  reducing  the  strength  than  small  rounded 
particles. 

Eutectic  Cast  Iron  vs.  the  Size  and  Form  of  the  Graphite  Particles.  —  It  seems 
probable,  as  more  fully  explained  in  Chapter  XXVI,  that  the  bulk  of  the  graphite 
present  in  cast  iron  forms  during  its  solidification  and  since  eutectic  cast  iron  solid- 
ifies quickly  while  its  temperature  remains  constant  whereas  the  solidification  of 
hypo-  and  hyper-eutectic  alloys  cover  wider  ranges  of  falling  temperature  it  seem? 
reasonable  to  infer  as  already  pointed  out  that  in  eutectic  cast  iron  the  graphite 
must  occur  in  smaller  and  more  rounded  particles.  In  other  words  the  mushy, 
semi-fluid  condition  of  relatively  long  duration  assumed  by  non-eutectic  alloys  on 
solidifying  must  promote  the  formation  of  coarse  graphite  particles.  If  this  view- 
is  correct  it  follows  that  for  otherwise  like  composition  and  like  treatment,  cast  iron 
of  eutectic  composition  should  be  the  strongest.  In  following  that  line  of  thought 
the  influence  exerted  by  some  impurities  on  the  eutectic  carbon  ratio  should  not  be 
overlooked.  With  some  2  to  3  per  cent  silicon,  for  instance,  some  3.50  to  3.75  per 
cent  carbon  should  yield  the  eutectic  alloy. 

Silicon  and  Rate  of  Cooling  vs.  the  Matrix  of  Cast  Iron  and  the  Formation  of 
Graphite. —  It  is  well  known  that  the  presence  of  silicon  in  cast  iron  promotes  the  for- 
mation of  graphitic  carbon  while  a  rapid  rate  of  cooling  during  solidification  oppose  •- 
it  and  causes  it  to  occur  in  particles  of  smaller  size. 

The  combined  action  of  the  rate  of  cooling  and  of  the  silicon  content  in  determin- 
ing the  character  of  the  matrix  of  cast  iron  may  be  advantageously  shown  graphi- 
cally as  in  Figure  362.  The  abscissae  correspond  to  percentages  of  silicon,  the  ordinates 
to  rates  of  cooling  represented  by  arbitrary  numbers,  2  indicating  quicker  cooling 
than  1,  3  quicker  cooling  than  2,  etc.  Any  point  on  the  line  A  B  corresponds  to  a 
set  of  conditions  producing  a  eutectoid  matrix,  while  any  point  above  A  B  refers  to 
rates  of  cooling  and  silicon  contents  resulting  in  hyper-eutectoid,  and  below  AB,  in 
hypo-eutectoid  matrix.  The  point  M,  for  instance,  on  A  B  corresponds  to  1  per 
cent  of  Si  and  a  rate  of  cooling  represented  by  1.5,  while  N  corresponds  to  3  per  cent 
of  Si  and  a  quicker  rate  of  cooling,  4.4.  Both  sets  of  conditions  will  produce  castings 
having  a  eutectoid  matrix  but  not  identical  properties,  because  the  quicker  cooling  in 
the  latter  case  must  produce  smaller  and  less  angular  graphite  particles  and  hence  a 
stronger  metal. 

Cast  Iron  of  Maximum  Strength.  — •  From  the  foregoing  considerations  the  follow- 
ing conclusions  may  be  drawn:  (1)  to  possess  maximum  strength,  cast  iron  should  be 


CHAPTER   XXII  — CAST   IRON 


381 


<>f  rutectic  composition  because  the  graphite  particles  will  then  be  smaller  which  in 
turn  makes  for  greater  strength,  (2)  the  eutectic  structure  should  be  produced  by 
high  silicon  and  low  carbon  content  since  the  latter  promotes  strength,  (3)  the  cast 
iron  should  have  a  eutectoid  matrix  because  it  is  the  matrix  of  greatest  strength, 
and  (4)  the  eutectoid  matrix  should  be  produced  by  high  silicon  content  and  rapid 
cooling  because  the  latter  leads  to  the  formation  of  small  graphite  particles. 

Solidification  of  Eutectic  Cast  Iron.  • —  The  mechanism  of  the  solidification  of 
eutectic  cast  iron  and  of  its  subsequent  transformation  on  slow  cooling  may  be  il- 
lustrated graphically  as  shown  in  Figure  363,  assuming  that  no  graphite  forms.  A  BCD 
represents  a  block  of  eutectic  cast  iron  cooling  from  above  its  melting-point  to  at- 
mospheric temperature.  Above  its  liquidus-solidus  line  EF  the  metal  is  liquid.  On 


PER  CENT  SILICON 


Fig.  362.  —  Influence  of  silicon-content  and  of  rate  of  cooling  in 
determining  the  character  of  the  matrix  of  cast  iron. 


reaching  that  lire,  which  in  pure  iron-carbon  alloys  corresponds  to  a  temperature  of 
1130  deg.  C.,  it  solidifies  as  an  austenite-cementite  eutectic  containing  EG  (47.7  per 
cent)  of  austenite  and  GF  (52.3  per  cent)  of  cementite  as  later  explained.  On  cool- 
ing from  the  solidus  to  the  eutectoid  line  HL  or  Ari  point,  at  about  700  deg.  C., 
the  eutectic  austenite  rejects  cementite  (pro-eutectoid  cementite)  until  it  reaches  the 
eutectoid  composition  (0.85  per  cent  of  C  or  12.75  per  cent  of  Fe3C).  The  pro- 
eutectoid  cementite  thus  formed  joins  the  eutectic  cementite  and  is  represented  by 
the  triangle  GIK  in  the  diagram.  Clearly  IL  represents  the  total  free  cementite  in 
the  alloy  when  it  reaches  its  eutectoid  point.  On  cooling  through  this  point  the  re- 
maining austenite,  now  of  eutectoid  composition,  breaks  up  into  ferrite  and  cemen- 
tite to  form  pearlite,  a  constituent  which  contains  about  12.75  per  cent  of  cementite 
(01  in  the  diagram)  and  87.25  per  cent  of  ferrite  (HO  in  the  diagram).  Finally,  then, 
the  completely  cooled  alloy  contains  ND  per  cent  of  cementite  of  eutectic  origin, 
MN  per  cent  of  cementite  of  pro-eutectoid  origin,  PM  per  cent  of  eutectoid  cemen- 
tite and  CP  per  cent  of  ferrite.  The  eutectic  and  pro-eutectoid  cementite  known 


382  CHAPTER  XXII  — CAST  IRON 

collectively  as  "free"  or  "excess"  cementite  (MD)  are  so  merged  that  they  cannot 
be  distinguished  from  each  other,  while  the  eutectoid  cementite  exists  as  a 
distinct  constituent  of  pearlite.  In  the  diagram  PD  represents  the  total  cementite 
(free  and  eutectoid)  in  the  alloy  and  CP  the  ferrite,  that  is  its  ultimate  structural 
composition. 

Solidification  of  Hyper-Eutectic  Cast  Iron.  —  The  mechanism  of  the  solidifi- 
cation of  hyper-eutectic  cast  iron  and  of  its  subsequent  transformation  on  slow  cool- 
ing may  be  represented  graphically,  as  shown  in  Figure  364.  It  is  assumed  in  this 
diagram  that  no  graphitic  carbon  is  formed. 

On  reaching  its  liquidus,  EF,  in  the  vicinity  of  1,200  deg.  C.  in  the  case  of  a  pure 
alloy  containing  5  per  cent  of  carbon,  cementite  begins  to  form,  and  its  formation 
continues  to  the  solidus  line,  as  indicated  by  the  triangle  FGH.  On  reaching  the 
solidus  (1130  deg.  C.)  the  molten  portion  of  the  alloy  has  the  eutectic  composition 
(4.30  per  cent  of  carbon),  and  it  solidifies  as  an  austenite-cementite  eutectic,  IG, 
containing  IK  per  cent  of  austenite  and  KG  per  cent  of  cementite.  This  is  in  ac- 
cordance with  the  solidification  of  the  molten  solution  of  any  two  metals  capable 
of  forming  a  eutectic  alloy  as  explained  in  Chapters  XXV  and  XXVI.  On  cooling 
from  its  solidus  to  its  eutectoid  or  Ari  point  (700  deg.  C.)  the  eutectic  austenite 
rejects  cementite,  as  indicated  by  the  triangle  KQN,  until,  on  reaching  the  eutectoid 
point,  it  is  of  eutectoid  composition,  and  is  converted  into  pearlite  containing  LM 
per  cent  of  ferrite  and  MQ  per  cent  of  cementite.  The  alloy  will  finally  consist  of 
UD  per  cent  of  pro-eutectic  cementite,  TU  per  cent  of  eutectic  cementite,  ST  per 
cent  of  pro-eutectoid  cementite,  RS  per  cent  of  eutectoid  cementite,  and  CR  per 
cent  of  ferrite.  The  pro-eutectic,  eutectic,  and  pro-eutectoid  cementites  are  merged, 
and  cannot  be  distinguished  from  each  other  under  the  microscope.  Their  sum,  SD, 
constitutes  the  free  cementite  of  the  alloy.  The  eutectoid  cementite,  RS,  on  the 
contrary,  remains  intimately  associated  with  the  ferrite  to  form  the  constituent 
pearlite,  represented  by  CS  in  the  diagram.  RD  represents  the  total  percentage  of 
cementite  in  the  alloy. 

Solidification  of  Hypo-Eutectic  Cast  Iron.  —  The  mechanism  of  the  solidification 
of  hypo-eutectic  alloys  may  likewise  be  graphically  represented  as  shown  in  Figure  365. 
On  reaching  its  liquidus,  EF,  pro-eutectic  austenite  containing  1.70  per  cent  of  car- 
bon begins  to  form.  Assuming  the  cast  iron  to  contain  3.50  per  cent  of  carbon,  its 
temperature  would  then  be  in  the  vicinity  of  1225  deg.  C.  In  cooling  from  the 
liquidus  to  the  solidus  or  eutectic  temperature,  GK,  austenite  continues  to  form,  the 
percentage  of  that  constituent  solidifying  between  the  liquidus  and  the  solidus  be- 
ing represented  by  the  triangle  EGH.  On  reaching  the  eutectic  temperature,  the 
liquid  portion  of  the  alloy  has  reached  the  eutectic  composition  and  now  solidifies  as 
an  austenite-cementite  eutectic,  HK,  containing  HI  per  cent  of  austenite  and  IK 
per  cent  of  cementite  following  in  this  the  behavior  of  all  binary  alloys  in  which  a 
eutectic  alloy  is  formed  (Chapters  XXV  and  XXVI).  On  cooling  from  the  eutectic 
to  the  eutectoid  or  Ari  temperature,  both  the  pro-eutectic  and  the  eutectic  austenite 
reject  cementite,  as  indicated  by  the  triangle  IMN.  At  the  eutectoid  temperature, 
LO,  the  remaining  austenite  is  of  eutectoid  composition  and  is  transformed  into 
pearlite,  LM,  containing  LT  per  cent  of  ferrite  and  TM  per  cent  of  cementite.  Below 
the  eutectoid  temperature,  therefore,  the  cast  iron  consists  of  CR  per  cent  of  pearlite 
and  RD  per  cent  of  free  cementite,  the  pearlite  containing  CQ  per  cent  of  ferrite  and 
QR  per  cent  of  cementite,  and  the  free  cementite  RS  per  cent  of  cementite  of  eutectic 


CHAPTER   XXII— CAST   IRON 


383 


H    3 

£  I 

+a      V 
<*>    £ 

S3     -S  x 
I? 

il 


O      Q    <u 


.•s  § 

p  -  ft 


384  CHAPTER   XXII  — CAST   IRON 

origin,  and  SD  per  cent  of  cementite  of  pro-eutectoid  origin.  QD  evidently  represents 
the  total  cementite  in  the  alloy. 

The  Graphitizing  of  Cementite.  —  It  is  well  known  as  more  fully  explained  in 
Chapters  XXIV  and  XXVI  that  comontite  readily  breaks  up  into  iron  (ferrite)  and 
graphite  when  heated  to  a  sufficiently  high  temperature,  and  it  is  likewise  well  known 
that  the  higher  the  temperature  the  more  easily  does  this  dissociation  take  phici  . 
and  that  silicon  also  promotes  it.  Bearing  in  mind  the  influence  of  temperature  on 
the  graphitization  of  cementite,  we  are  justified  in  assuming  that  the  higher  the  tem- 
perature at  which  cementite  forms  the  more  readily  will  it  be  converted  into  ferrite 
and  graphite  on  subsequent  cooling.  It  follows  from  the  foregoing  assumption  that 
the  graphitization  of  the  cementite  formed  in  iron-carbon  alloys  during  and  below 
their  solidification  will  take  place  in  the  following  order  with  increasing  difficulty:  (1) 
graphitization  of  pro-eutectic  cementite,  (2)  graphitization  of  eutectic  cementite, 
(3)  graphitization  of  pro-eutectoid  cementite,  and  (4)  graphitization  of  eutectoid 
cementite. 

Graphitizing  of  Hyper-Eutectic  Alloys.  —  In  hyper-eutectic  alloys  pro-eutectic 
cementite  forms  as  soon  as  the  alloy  begins  to  solidify  and  keeps  on  forming  to  the 
eutectic  temperature.  At  such  high  temperature  the  dissociation  tendency  is  so  great 
that  graphite  very  readily  forms,  even  in  the  absence  of  silicon  and  during  relatively 
rapid  cooling.  If  time  be  given,  the  graphite  resulting  from  the  dissociation  of  pro- 
eutectic  cementite  rises  to  the  surface  of  the  still  liquid  bath,  when  it  is  known  as 
"kish."  Upon  reaching  the  eutectic  temperature,  eutectic  cementite  forms,  which 
easily  breaks  up  into  iron  and  graphite  if  the  cooling  be  sufficiently  slow,  its  graphi- 
tization being  assisted  by  the  presence  of  nuclei  of  pro-eutectic  graphite.  In  cooling 
from  the  eutectic  to  the  eutectoid  temperature  pro-eutectoid  cementite  is  expelled 
by  the  austenite.  The  graphitization  of  pro-eutectoid  cementite,  however,  proceeds 
with  less  readiness  because  of  the  relatively  low  temperature  now  prevailing,  although 
assisted  by  the  presence  of  many  nuclei  of  pro-eutectic  and  eutectic  graphite.  A 
large  percentage  of  silicon  and  very  slow  cooling  would  of  course  promote  it.  If 
some  of  the  pro-eutectoid  cementite  fails  to  be  graphitized  the  resulting  cast  iron 
will  necessarily  have  a  hyper-eutectoid  matrix  and  will  probably  be  mottled.  Upon 
reaching  the  eutectoid  temperature  eutectoid  cementite  is  formed,  but  its  graphitiza- 
tion, although  greatly  assisted  by  the  presence  of  much  graphite,  will  require,  at  least 
to  be  complete,  very  slow  cooling  and  the  presence  of  considerable  silicon.  It  is,  of 
course,  evident  that  if  some  pro-eutectoid  cementite  has  escaped  graphitization  none 
of  the  eutectoid  cementite  will  graphitize,  since  the  dissociation  of  cementite  becomes 
more  difficult  as  the  temperature  is  lowered.  In  case  the  graphitization  of  eutectoid 
cementite  is  complete,  a  rare  instance,  the  resulting  cast  iron  has  a  pure,  or  rather 
carbonless,  ferrite  matrix.  If  it  is  incomplete  the  matrix  will  be  hypo-eutectoid  and 
the  cast  iron  belong  to  one  of  the  foundry  grades. 

Graphitizing  of  Eutectic  Alloys.  —  On  reaching  the  solidification  temperature 
eutectic  cementite  is  formed,  which  breaks  up  quite  readily  into  ferrite  and  graphite 
unless  prevented  from  doing  so  through  rapid  cooling.  On  cooling  from  the  eutectic 
to  the  eutectoid  temperature  pro-eutectoid  cementite  forms,  the  graphitization  of 
which  is  promoted  by  the  presence  of  nuclei  of  eutectic  graphite  but  opposed  by  the 
lower  temperature  now  prevailing.  It  may,  however,  be  complete  in  the  presence  of 
sufficient  silicon.  If  it  remains  incomplete  the  cast  iron  will  have  a  hyper-eutectoid 
matrix.  The  graphitization  of  the  eutectoid  cementite  formed  at  the  eutectoid 


CHAPTER   XXII  — CAST   IRON  385 

temperature  is  much  less  readily  brought  about,  being  now  opposed  by  the  low  tem- 
perature of  the  metal.  It  will  generally  remain  incomplete  and  the  resulting  cast 
iron  will  have  a  hypo-eutectoid  matrix. 

Graphitizing  of  Hypo-Eutectic  Alloys.  —  In  hypo-eutectic  alloys  when  solidifi- 
cation begins  austenite  is  formed  and  at  the  eutectic  temperature  eutectic  cementite 
falls  out  of  solution.  This  cementite  undergoes  graphitization  quite  readily  because 
of  the  high  temperature  at  which  it  forms.  On  cooling  from  the  eutectic  to  the  eu- 
tectoid  temperature  pro-eutectoid  cementite  is  rejected  by  austenite,  and  its  graphi- 
tization proceeds  the  more  slowly  that  it  is  not  now  assisted  by  pre-existing  nuclei  of 
graphite  to  the  same  extent  as  eutectic  and  hyper-eutectic  alloys.  The  eutectoid 
cementite  is  graphitized  with  difficulty  because  of  low  temperature  and  of  relatively 
little  pre-existing  graphite.  If  the  graphitization  of  pro-eutectoid  cementite  remains 
incomplete  the  cast  iron  will  have  a  hyper-eutectoid  matrix,  while  in  case  of  complete 
graphitization  of  the  pro-eutectoid  cementite  but  incomplete  graphitization  of  eutec- 
toid cementite  it  will  have  a  hypo-eutectoid  matrix.  In  case  of  complete  graphiti- 
zation of  the  pro-eutectoid  cementite  and  of  no  graphitization  at  all  of  the  eutectoid 
cementite  the  cast  iron  would  have  a  eutectoid  matrix. 


CHAPTER  XXIII 

IMPURITIES   IN   CAST   IRON 

In  the  preceding  chapter  cast  iron  has  been  considered  as  a  pure  alloy  of  iron  and 
carbon,  but  like  steel,  commercial  cast  iron  always  contains  varying  proportions  of 
silicon,  manganese,  phosphorus,  and  sulphur,  and  we  should  now  examine  the  in- 
fluence of  these  impurities  on  its  structure  and  consequently  on  its  properties. 

Silicon  in  Cast  Iron.  —  Cast  iron  seldom  contains  less  than  0.50  per  cent  silicon 
and  frequently  as  much  as  3  or  3.50  per  cent.  As  in  the  case  of  steel  this  silicon  prob- 
ably combines  with  some  of  the  iron  to  form  the  silicide  of  iron,  FeSi,  which  is  then 
dissolved  in  the  balance  of  the  iron.  The  ferrite  of  cast  iron,  therefore,  always  holds 
a  considerable  amount  of  silicon  or  rather  of  the  silicide  FeSi  in  solution.  It  has  been 
seen  that  silicon  produces  exactly  three  times  its  own  weight  of  FeSi;  cast  iron  with 
2  per  cent  of  silicon,  for  instance,  will  contain  6  per  cent  of  FeSi  dissolved  in  its  fer- 
rite. The  influence  of  silicon  on  the  properties  of  cast  iron  is  very  important  chiefly 
through  its  promoting  the  formation  of  graphitic  carbon  and,  therefore,  increasing 
the  softness  and,  if  carried  too  far,  decreasing  the  strength  of  cast  iron.  It  is  why 
foundrymen  often  speak  of  silicon  as  a  "softener."  The  influence  of  silicon  in  caus- 
ing the  decomposition  of  the  carbide  FesC  is  ascribed  by  Stead,  Hatfield,  and  others 
to  the  presence  of  some  silicon  in  the  carbide  which  is  thereby  rendered  less  stable. 
That  silicon  increases  fluidity  while  decreasing  shrinkage  and  chill  is  also  well  known. 

The  influence  of  silicon  to  decrease  the  percentage  of  carbon  in  the  iron-carbon 
eutectic  has  been  considered  in  the  preceding  chapter. 

The  occurrence  of  varying  amounts  of  silicon  in  cast  iron  cannot  be  detected  under 
the  microscope,  unless  indirectly  and  roughly  through  the  presence  of  more  or  less 
graphitic  carbon.  It  is  probably  true,  however,  that  under  otherwise  similar  con- 
ditions, the  more  silicon  present  in  ferrite  the  slower  the  etching  of  that  constituent. 
The  ferrite  of  cast  iron  with  hypo-eutectoid  matrix,  for  instance,  possibly  because  of 
its  greater  silicon  content,  often  remains  brilliant  after  the  usual  deep  etching  treat- 
ment which  would  color  decidedly  some  of  the  grains  of  the  ferrite  of  wrought  iron 
or  of  low  carbon  steel. 

Sulphur  in  Cast  Iron.  —  Cast-iron  castings  of  good  quality  should  not  contain 
more  than  0.1  per  cent  of  sulphur  while  but  a  trace  of  that  element  may  be  present. 
In  the  manufacture  of  chilled  castings,  however,  as  much  as  0.2  per  cent  sulphur  is 
sometimes  allowed.  It  has  been  explained  in  Chapter  IX  that  because  of  the  great 
affinity  of  manganese  fdr  sulphur  these  two  elements  readily  combine  to  form  the 
manganese  sulphide  MnS,  each  part  by  weight  of  sulphur  giving  rise  to  the  forma- 
tion approximately  of  2J/2  parts  of  MnS.  Cast  iron  with  0.05  per  cent  sulphur,  for 
instance,  would  contain  about  0.125  per  cent  of  MnS.  As  in  the  case  of  steel  this 

386 


CHAPTER   XXIII  —  IMPURITIES   IN    CAST   IRON 


387 


sulphide  occurs  in  the  form  of  rounded  particles  of  a  dove  gray  or  slate  color  embedded 
in  the  metallic  matrix  (Fig.  366).  Should  there  not  be  enough  manganese  present  to 
combine  with  all  the  sulphur,  the  remaining  sulphur  would  unite  with  iron  to  form 
the  sulphide  FeS  which  would  occur  as  rounded  yellow  areas.  It  should  be  noted, 
however,  that  since  it  requires  less  than  2  parts  by  weight  of  manganese  to  combine 
with  one  part  of  sulphur,  when  cast  iron  contains  twice  as  much  manganese  as  it  does 
sulphur  no  iron  sulphide  can  be  formed,  theoretically  at  least.  Since  it  is  seldom  that 
cast  iron  does  not  contain  a  considerably  greater  proportion  of  manganese  the  occur- 
rence of  FeS  in  cast  iron  should  be  rare. 

The  influence  of  sulphur  in  opposing  the  formation  of  graphitic  carbon  has  already 
been  mentioned;  it  may  consequently  be  said  to  harden  cast  iron.    Hatfield  believes 


Fig.  3tiO.  —  Partially  inalleablized  cast  iron.  Magnified  670  diameters.  Sul- 
phur about  0.2  per  cent,  manganese  0.50  per  cent.  (C.  C.  Buck,  Cor- 
respondent Course  student.) 


that  this  influence  of  sulphur  is  due  to  the  presence  of  some  sulphide  of  iron  dissolved 
in  the  iron  carbide  and  increasing  its  stability.  It  has  also  a  well-known  influence  in 
increasing  the  depth  of  "chill"  in  solidifying  cast  iron  against  a  metal  wall,  that  is 
the  thickness  of  metal  free  from  graphitic  carbon  produced  by  the  cooling  action  of 
that  wall.  Its  other  influences  are  harmful  as  it  increases  shrinkage,  causes  the  molten 
metal  to  be  sluggish,  and  induces  unsoundness. 

Manganese  in  Cast  Iron.  —  Special  cast  irons  are  made,  known  as  spiegeleisen, 
ferro-manganese,  etc.,  containing  very  large  proportions  of  manganese,  but  in  ordi- 
nary castings  the  amount  of  manganese  seldom  exceeds  2  per  cent  and  may  be  as  low 
as  0.10  per  cent.  It  has  been  shown  in  Chapter  IX  that  when  manganese  is  present  in 
these  relatively  small  proportions  it  first  combines  with  sulphur  to  form  the  sulphide 
MnS,  and  then  with  carbon  to  form  the  carbide  Mn3C,  this  carbide  uniting  with  the 


388  CHAPTER   XXIII  —  IMPURITIES   IN   CAST   IRON 

carbide  Fe3C  to  form  cementite.  The  cementite  of  cast  iron,  therefore,  like  that  of 
steel  nearly  always  contains  some  Mn3C.  Since  iron  and  manganese,  however,  have 
nearly  the  same  atomic  weights  it  remains  true  that  to  obtain  the  percentage  of  cemen- 
tite in  any  commercial  iron-carbon  alloy  it  suffices  to  multiply  its  percentage  of  com- 
bined carbon  by  fifteen. 

The  influence  of  manganese  in  opposing  the  formation  of  graphitic  carbon  has 
already  been  noted.  It  is  explained  by  some  on  the  ground  that  the  carbide  Mn3C  is 
more  stable  than  Fe3C.  Like  sulphur  manganese  is  a  hardener,  its  presence  in  large 
proportions  increasing  the  difficulty  of  machining  castings.  It  promotes  the  absorp- 
tion of  carbon  by  iron.  Some  believe  that  it  increases  shrinkage  and  that  while  it 
has  no  marked  influence  on  the  depth  of  the  chill  it  increases  its  hardness. 


Fig.  367.  —  Alloy  of  iron  and  phosphorus.  Fig.  368.  — •  Alloy  of  iron  and  phosphorus. 
Phosphorus  1.8  per  cent.  Magnified  350  Phosphorus  8  per  cent.  Magnified  250 
diameters.  (Stead.)  diameters.  (Stead.) 


Phosphorus  in  Cast  Iron.  —  It  has  been  explained  in  Chapter  XX  that  when 
phosphorus  occurs  in  very  small  quantities  as  it  does  in  steel,  the  totality  of  it  prob- 
ably forms  the  phosphide  Fe3P,  which  is  then  dissolved  by  the  iron.  In  cast  iron, 
however,  because  of  the  frequent  presence  of  a  considerable  proportion  of  phos- 
phorus and  of  a  larger  proportion  of  carbon  this  element  assumes  another  condition. 
The  occurrence  of  phosphorus  in  cast  iron  was  first  studied  and  described  by  Stead. 
The  results  of  his  important  investigations  are  briefly  summarized  below : 

(1)  When  phosphorus  is  alloyed  with  carbonless  iron  in  amounts  varying  from 
traces  to  1.70  per  cent,  it  forms  a  phosphide  corresponding  to  the  formula  Fe3P, 
which  is  held  in  solid  solution  by  the  iron.    All  the  metals  used  commercially,  such 
as  wrought  iron  and  steels  containing  very  little  carbon,  may  be  included  in  this 
class.     They  consist  essentially  of  polyhedral  grains  of  ferrite  holding  Fe3P   in 
solution. 

(2)  When  the  metal  contains  from  1.70  to  10.2  per  cent  phosphorus  it  consists  of 
a  saturated  solution  of  Fe3P  in  iron  (1.70  per  cent  P)  and  of  a  eutectic  alloy  contain- 


CHAPTKR  XXIII  — IMPURITIES   IN   CAST   IRON 


389 


ing  about  10.2  per  pent  P  and  made  up  of  about  61  per  cent  FesP  and  39  per  cent  of 
the  saturated  solution  of  Fe3P  in  iron.  To  account  readily  for  this  structure  and  that 
of  the  following  groups,  it  is  only  necessary  to  consider  these  metals  as  alloys  of  two 
constituents:  one  the  phosphide  Fe3P,  and  the  other  a  saturated  solution  of  Fe3P  in 
iron.  It  is  well  known  that  a  certain  class  of  binary  alloys  when  solidifying  give  rise 
to  the  formation  of  a  eutectic  alloy,  that  is,  of  a  mechanical  mixture  made  up  in  defi- 
nite proportions,  of  extremely  small  plates  alternately  of  one  and  the  other  constit- 
uents, the  balance  of  the  mass  consisting  of  that  constituent  which  is  present  in 
excess  over  the  amount  required  to  form  the  eutectic  alloy.  ~Tt~is  precisely  what 
happens  in  the  case  of  iron  containing  over  1.70  per  cent  phosphorus.  The  formation 
of  eutectic  alloys  will  be  further  described  in  Chapter  XXV. 

Figures  367,  368,  and  369  illustrate  the  structure  of  iron  containing  respectively 
1.8,  8,  and  10.2  per  cent  phosphorus.    The  mottled  constituent  made  up  of  two  struc- 


\ 


Fig.  369.  —  Alloy  of  iron  and  phosphorus. 
Phosphorus  10.2  per  cent.  Magnified  350 
diameters.  (Stead.) 


Fig.  370.  —  Alloy  of  iron  and  phosphorus. 
Phosphorus  11.07  per  cent.  Magnified  GO 
diameters.  (Stead.) 


tural  elements  in  close  juxtaposition  corresponds  in  every  case  to  the  phosphide 
eutectic.  The  background  of  Figure  367  and  the  clear  regions  of  Figure  368  are  com- 
posed of  the  solid  saturated  solution,  while  Figure  369  is  composed  entirely  of  the 
eutectic  alloy. 

(3)  When  the  iron  contains  from  10.2  per  cent  to  15.58  per  cent  phosphorus  it  is 
composed  of  crystals  of  FesP  surrounded  by  the  eutectic  mixture  just  described,  as 
illustrated  in  Figure  370,  in  which  the  white  angular  areas  represent  Fe3P  and  the 
background  the  eutectic  alloy. 

When  carbon  is  present  in  considerable  quantity  as  in  cast  iron,  the  phosphorus 
held  in  solution  by  the  iron  in  the  absence  of  carbon  is  thrown  out  of  solution 
almost  completely  and  a  ternary  eutectic  formed  (Stead,  Wiist,  Gcerens)  containing 
according  to  Stead  91.19  per  cent  iron,  6.89  per  cent  phosphorus,  and  1.92  per  cent 
carbon,  its  proximate  constituents  being  iron  carbide  (Fe3C),  iron  phosphide  (Fe3P), 


390 


CHAPTER   XXIII  —  IMPURITIES   IN   CAST   IRON 


and  iron  (ferrite)  containing  a  little  phosphorus  in  solution.  After  heat  tinting  the 
constituents  of  the  ternary  eutectic  are  readily  distinguishable,  the  iron  carbide  being 
colored  red,  the  iron  phosphide  purple,  and  the  ferrite  containing  some  phosphorus 


Fig.  371.  — Alloy  of  iron,  phosphorus,  and  car- 
bon. Phosphorus  1.74  per  cent,  carbon  0.18 
per  cent.  Magnified  60  diameters.  (Stead.) 


Fig.  372.  —  Alloy  of  iron,  phosphorus,  and  car- 
bon. Phosphorus  1.70  per  cent,  carbon  0.71 
per  cent.  Magnified  250  diameters.  (Stead.) 


Fig.  373.  —  Alloy  of  iron,  phosphorus,  and  car- 
bon. Phosphorus  1.70  per  cent,  carbon  1.40 
per  cent.  Magnified  250  diameters.  (Stead.) 


in  solution,  white.  While  such  is  the  character  of  the  phosphide  eutectic  in  white 
and  mottled  cast  iron,  Stead  writes  that  in  very  gray  phosphoretic  metals  the  car- 
bon diffuses  out  of  it  and  a  binary  eutectic  of  Fe3P  and  iron  containing  in  solution 


CHAPTER   XXIII— IMPURITIES   IN   CAST   IRON  391 

a  little  phosphorus  is  formed.  For  this  binary  phosphide  eutectic  of  gray  cast  iron 
first  described  by  Stead  in  1900  the  author  proposed  the  name  of  "steadite."  It  con- 
tains according  to  Stead  about  10  per  cent  phosphorus  and  90  per  cent  iron. 

The  structures  of  some  iron  alloys  containing  1.70  per  cent  phosphorus  and  in- 
creasing proportions  of  carbon  are  illustrated  in  Figures  371  to  373.  The  increasing 
proportion  of  phosphorus  thrown  out  of  solution  as  a  eutectic  will  be  noted. 

It  will  be  obvious  from  the  above  description  of  the  behavior  of  phosphorus  that 
in  cast  iron,  because  of  the  presence  of  a  large  amount  of  carbon,  nearly  the  whole  of 
the  phosphorus  is  liberated  from  its  solution  with  iron  and  causedto  occur  as  a  ter- 
nary eutectic  in  white  and  mottled  cast  iron  and  as  a  binary  eutectic  or  steadite  in 
gray  cast  iron,  even  if  the  metal  contains  less  than  the  necessary  amount  of  phos- 
phorus needed  to  saturate  the  iron,  namely  1.70  per  cent.  Indeed  so  marked  is  this 
action  of  carbon  that  Stead  tells  us  that  in  steel  containing  but  0.1  per  cent  phos- 
phorus a  portion  of  it  is  liable  to  be  thrown  out  of  solution  in  the  presence  of  0.90  per 
cent  carbon. 

To  sum  up,  the  phosphorus  in  ordinary  steels  occurs  chiefly  and  probably  alto- 
gether as  the  phosphide  Fe3P  dissolved  in  iron  while  in  cast  iron  it  occurs  chiefly  if 
not  entirely  as  a  eutectic.  While  Stead  writes  that  the  whole  of  the  phosphorus  is 
not  liberated  from  solution  even  in  the  presence  of  much  carbon,  the  amount  re- 
tained in  solution  in  the  presence  of  some  3  per  cent  or  more  carbon  is  apparently 
very  small  and  it  may  probably  be  assumed  for  all  practical  purposes  that  in  cast 
iron  the  whole  of  the  phosphorus  is  present  as  a  eutectic,  for  Stead  says  that  the 
phosphide  eutectic  may  be  detected  in  cast  iron  containing  as  little  as  0.03  per  cent 
phosphorus.  The  structure  of  gray  phosphoretic  cast  iron  is  illustrated  in  Figures 
374  and  377.  The  various  constituents,  graphite,  pearlite,  free  ferrite,  and  steadite 
are  easily  distinguishable  as  follows:  dark  plates,  graphite;  dark,  rounded  constit- 
uent, pearlite;  white,  structureless,  ferrite;  white,  mottled  (dotted),  steadite.  The 
structure  of  the  phosphide  eutectic  is  shown  more  highly  magnified  in  Figure  375. 
The  matrix  of  the  cast  iron  illustrated  in  Figures  374  and  375  is  free  from  com- 
bined carbon,  consisting,  therefore,  exclusively  of  ferrite  and  steadite;  in  Figure  376 
the  matrix  contains  both  considerable  pearlite  and  free  ferrite;  the  cast  iron 
shown  in  Figure  377  has  a  eutectoid  matrix. 

Stead  recommends  heat  tinting  as  a  suitable  treatment  for  bringing  out  the 
phosphide  eutectic,  especially  in  white  cast  iron  when  there  is  danger  of  confound- 
ing it  with  cementite.  The  heat-tinting  method  of  Stead  has  been  described  in 
Chapter  II. 

Stead  explains  as  follows  the  fact  that  a  relatively  high  proportion  of  phosphorus 
in  cast  iron  does  not  produce  extreme  brittleness. 

"The  reason  why  phosphoretic  pig  irons  are  not  more  brittle  than  they  are  is 
because  the  eutectic  separates  into  isolated  segregations,  and  does  not  form  con- 
tinuous cells  round  the  crystalline  grains.  When  the  phosphorus  does  not  exceed 
1.7  per  cent  the  metal  is  comparatively  strong,  but  an  addition  of  0.3  per  cent  re- 
duces the  strength  materially,  the  reason  of  which  is  that  the  eutectic  brittle  areas 
in  metal  with  2  per  cent  phosphorus  approach  each  other  closely,  leaving  less  of  the 
strong  ground  mass  intervening." 

Phosphorus  has  no  marked  influence  upon  the  condition  in  which  carbon  occurs 
in  cast  iron  but  it  increases  the  fluidity  of  the  metal  probably  because  of  the  forma- 
tion of  a  large  quantity  of  fusible  and  fluid  phosphide  eutectic. 


392 


CHAPTER   XXIII  —  IMPURITIES   IN   CAST   IROX 


Fig.  374.  —  Cast  iron.  Graphitic  carbon  3.06  per  cent.  Combined  carbon 
0.08  per  cent.  Phosphorus  1.36  per  cent.  Magnified  100  diameters. 
(F.  H.  Franklin  of  Saunders  and  Franklin.) 


Fig.  375.  —  Same  as  Fig.  374  but  magnified  425  diameters.    (F.  II.  Franklin 
of  Saunders  and  Franklin.) 


CHAPTER   XXIII  —  IMPURITIES   IN   CAST   IRON 


393 


Fig.  376.  —  Cast  iron.    Magnified  100  diameters.    Graph- 
ite, f pi-rite,  pearlite,  and  steadite.     (Boylston.) 


Fig.  377  —  Cast  iron.     Eutectoid  matrix.     Phosphorus  1.36  per  cent.     Mag- 
nified 100  diameters.     (F.  H.  Franklin  of  Saunders  and  Franklin.) 


394 


CHAPTER   XXIII  —  IMPURITIES   IN   CAST    IRON 


Critical  Points  of  Cast  Iron  Containing  Phosphorus.  —  H.  I.  Coe  has  published 
the  interesting  and  significant  cooling  curves  of  cast  iron  containing  increasing  pro- 
portions of  phosphorus  reproduced  in  Figure  378.  In  the  six  samples  of  cast  iron  in- 
volved the  percentages  of  phosphorus  were  respectively  0.09,  0.53,  1.01,  1.23,  2.16, 
and  2.90.  The  two  upper  points  correspond  to  the  solidification  of  the  iron-carbon 
alloys,  as  explained  in  Chapters  XXV  and  XXVI,  the  third  point  to  the  solidifica- 
tion of  the  phosphide  eutectic1  and  the  fourth  to  the  recalescence  point  (pear lite 
formation).  It  is  to  be  inferred  from  these  curves  that  as  phosphorus  increases  (a) 
the  melting-point  of  the  alloy  is  lowered,  (6)  the  intensity  of  the  third  point  increases, 
(c)  the  position  of  the  third  point  is  slightly  raised,  and  (d)  the  position  of  the  point 
of  recalescence  remains  practically  unaffected. 


Fig.  378.  —  Cooling  curves   of  cast-iron  samples  containing  increasing  per- 
centages of  phosphorus.     (H.  I.  Coe.) 


Structural  Composition  of  Phosphoretic  Cast  Iron.  —  Since  the  presence  of  10.2 
per  cent  phosphorus  causes  the  production  of  100  per  cent  steadite  it  follows  that 
the  phosphorus  in  cast  iron  gives  rise  to  the  formation  of  approximately  10  times  its 
own  weight  of  steadite. 

In  calculating  the  structural  composition  of  cast  iron,  therefore,  the  amount  of 
phosphorus  present  must  be  considered  as  it  may  very  materially  lower  the  percent- 
age of  combined  carbon  needed  to  convert  its  matrix  into  pearlite.  Bearing  in  mind 
that  cast  iron  to  be  free  from  both  free  ferrite  and  free  cementite  must  contain  ferrite 
and  cementite  in  the  proportion  of  seven  to  one  the  following  relation  will  indicate 
the  needed  amount  of  combined  carbon : 

Ferrite  =  100  -  G  -  lOPh  -  15C  =  7  X  15C  =  105C 


cementite 
or  100  -G  -  lOPh  -  120C  =  0 

Stead  indicates  980  to  1000  deg.  C.  as  the-  solidification  point  of  the  phosphide  eutectic. 


CHAPTER  XXIII  —  IMPURITIES   IN   CAST   IRON  395 

in  which  G,  C,  and  Ph  represent  respectively  the  percentage  of  graphite,  combined 
carbon,  and  phosphorus,  15C  representing,  of  course,  the  proportion  of  cementite 
and  lOPh  that  of  steadite. 

If  the  first  term  of  the  above  equation  is  greater  than  0  the  metal  will  contain  free 
ferrite,  i.e.  its  matrix  will  be  hypo-eutectoid;  if  it  is  smaller  than  0  it  will  contain  pure 
cementite,  i.e.  its  matrix  will  be  hyper-eutectoid. 

In  the  presence  of  3  per  cent  of  graphite  and  1  per  cent  phosphorus,  for  instance, 
the  percentage  of  combined  carbon  needed  to  produce  a  eutectoid  matrix  will  readily 
be  obtained  in  solving  the  equation 

100  -  3  -  10  -  120C  =  0 

which  calls  for  0.75  per  cent  combined  carbon.  Loss  combined  carbon  would  pro- 
duce free  ferrite  while  more  would  cause  the  formation  of  free  cementite. 

In  calculating  the  structural  composition  of  any  phosphoretic  cast  iron  of  known 
percentage  of  phosphorus,  graphite,  and  combined  carbon,  it  should  first  be  ascer- 
tained therefore  whether  its  matrix  will  be  hypo-  or  hyper-eutectoid.  Let  us  assume, 
for  instance,  a  cast  iron  containing  1.50  per  cent  phosphorus,  3.25  per  cent  graphite, 
and  0.40  per  cent  combined  carbon.  Since  100  —  3.25  —  15  —  120  X  0.40  is  greater 
than  0  the  matrix  of  the  iron  will  be  hypo-eutectoid,  that  is  the  metal  will  contain 
free  ferrite.  The  following  equations  will  then  permit  the  ready  calculation  of  its 
structural  composition. 

(1)P  +  F  +  S  +  G  =  100 

(2)  P  =  120C 

(3)  S  =  lOPh 
which  give 

Pearlite  (P)  =  48.00  per  cent 

Free  ferrite  (F)  ==  33.75  "  " 
Steadite  (S)  =  15.00  "  " 
Graphite  (G)  3.25  "  " 

100.00 

Taking  another  example,  a  cast  iron  containing  2  per  cent  graphite,  1.50  per  cent 
combined  carbon,  and  0.75  per  cent  of  phosphorus,  since  100  —  2  —  7.50  —  120  XI. 50 
is  less  than  0,  the  iron  will  contain  free  cementite.  Its  structural  composition  will  be 
obtained  by  solving  the  equations 

(1)  P  +  Cm  +  S  +  G  =  100 

(2)  P  =  ?  (100  -  15C  -  lOPh  -  G) 

(3)  S  =  lOPh 
which  give 

Pearlite  (P)  =  77.78  per  cent 

Free  cementite  (Cm)    =  12.72    "      " 
Steadite  (S)  =    7.50   "      " 

Graphite  (G)  2.00    "      " 

100.00 

Chemical  vs.  Structural  Composition.  —  It  will  now  be  instructive  to  compare  the 
chemical  composition  of  cast  iron  both  ultimate  and  proximate  with  its  structural 


396  CHAPTER  XXIII  —  IMPURITIES   IN   CAST   IRON 

composition.    To  that  effect  let  us  assume  a  cast  iron  of  the  following  ultimate  chemi- 
cal composition: 

Graphitic  carbon  3.00  per  cent 

Combined  carbon  0.50    "      " 

Silicon  2.00    "      " 

Phosphorus  1.50    "       " 

Manganese  0.40    "      " 

Sulphur  0.02    "      " 

Iron  (by  difference)     92.58    "      " 
100.00 

In  view  of  the  foregoing  considerations  the  following  proximate  compounds  will 
be  formed: 

(1)  0.02  per  cent  S  will  produce  about  0.05  per  cent  MnS. 

(2)  0.05  per  cent  MnS  contains  about  0.03  per  cent  Mn. 

(3)  This  leaves  0.40  —  0.03  =  0.37  per  cent  Mn  in  excess  to  combine  with  carbon. 

(4)  0.37  per  cent  Mn  will  form  0.39  per  cent  Mn3C. 

(5)  0.39  per  cent  Mn3C  contains  about  0.02  per  cent  carbon. 

(6)  This  leaves  0.50  —  0.02  =  0.48  per  cent  carbon  to  combine  with  iron. 

(7)  0.48  per  cent  carbon  will  form  7.20  per  cent  Fe3C. 

(8)  1.50  per  cent  phosphorus  will  form  9.63  per  cent  Fe3P. 

(9)  2.00  per  cent  silicon  will  form  6.00  per  cent  FeSi. 

The  proximate  chemical  analysis  of  the  cast  iron  considered  will  consequently  be: 

Graphitic  carbon  3.00  per  cent 

Fe3C  7.20  "  " 

Mn3C  0.39  "  " 

Fe3P  9.63  "  " 

FeSi  6.00  "  " 

MnS  0.05  "  " 

Iron  (by  difference)  73.73  "  " 
100.00 

Knowing  the  proximate  chemical  constituents  the  structural  composition  can  be 
readily  calculated.  The  Fe3C  and  Mn3C  form  the  cementite,  hence  the  cast  iron 
contains  7.20  +  0.39  =  7.59  per  cent  cementite.  The  whole  of  this  cementite,  since 
the  iron  evidently  has  an  hypo-eutectoid  matrix,  will  combine  with  ferrite  in  the 
proportion  of  7  to  1  to  form  pearlite,  hence  the  cast  iron  will  contain  7.59  X  8  =  60.72 
per  cent  pearlite;  1.50  per  cent  phosphorus  means  15.00  per  cent  of  steadite;  the  6.00 
per  cent  of  FeSi  will  be  dissolved  in  the  ferrite  while  the  small  quantity  of  MnS  will 
occur  as  independent  minute  particles.  The  structural  composition  will  therefore  be: 

Pearlite  (P)  =  60.72  per  cent 

Free  ferrite  (F)  (by  difference)     =  21.23    "      " 
Steadite  (S)  =  15.00    "      " 

Graphite  (G)  3.00    "      " 

MnS  0.05    "      " 

100.00 


CHAPTER  XXIII  —  IMPURITIES   IN   CAST   IRON  397 

The  quicker  method  for  the  calculation  of  the  structural  composition  of  cast  iron 
given  in  foregoing  pages,  and  in  which  the  presence  of  manganese,  silicon,  and  sul- 
phur is  ignored  would  have  given : 

Pearlite  (P)  60.00 

Free  ferrite  (F)       22.00 
Steadite  (S)  15.00 

Graphite  (G)  3.00 

100.00 

For  practical  purposes  these  values  are  identical  to  those  obtained  from  a  knowledge 
of  the  complete  analysis  of  the  iron. 

Other  Impurities.  —  Nickel  in  appreciable  quantity  seldom  occurs  in  cast  iron. 
It  is  believed  to  form  a  solid  solution  with  ferrite  and  to  promote  the  formation  of 
graphitic  carbon.  Vanadium  probably  strengthens  cast  iron,  an  influence  which  is 
explained  by  W.  H.  Hatfield  on  the  ground  that  the  bulk  of  the  vanadium  crystal- 
lizes with  the  iron  carbide  which  is  thereby  rendered  more  stable.  J.  E.  Johnson 
has  recently  contended  that  oxygen  strengthens  and  otherwise  improves  cast  iron 
while  Moldenke  blames  oxygen  for  many  of  the  ailments  from  which  it  occasionally 
suffers.  Many  doubt  the  presence  of  any  appreciable  amount  of  oxygen  in  cast 
iron. 


CHAPTER  XXIV 

MALLEABLE   CAST   IRON 

Graphitizing  of  Cementite.  —  The  unstability  of  cementite  has  already  been  al- 
luded to.  It  has  been  mentioned  that  the  prolonged  annealing  of  high  carbon  steel 
above  its  critical  range  was  always  likely  to  result  in  the  formation  of  some  graphitic- 
carbon  through  the  dissociation  of  the  unstable  cementite  according  to  the  reaction : 

Fe3C  =  3  Fe  +  C 

The  graphitic  carbon  formed  in  this  way  is  often  called,  according  to  Ledebur 
"temper"  carbon  to  distinguish  it  from  the  graphite  formed  during  the  solidification 
of  cast  iron.  This  breaking  up  of  cementite  into  ferrite  and  graphite  takes  place  the 
more  readily  (1)  the  more  combined  carbon  in  the  metal,  (2)  the  higher  the  tempera- 
ture, (3)  the  longer  the  exposure  to  a  high  temperature,  (4)  the  more  silicon  and  the 
less  manganese  and  sulphur  present.  The  influence  of  a  large  amount  of  combined 
carbon  in  promoting  the  graphitizing  of  cementite  is  made  evident  by  the  facts  (1) 
that  iron-carbon  alloys  containing  less  than  some  0.50  per  cent  carbon  cannot  be 
made  graphitic  under  the  most  favorable  annealing  conditions  even  when  containing 
much  silicon,  (2)  that  in  steel  containing  in  the  vicinity  of  one  per  cent  carbon  the 
graphitizing  proceeds  very  slowly  and  remains  partial,  and  (3)  that  in  alloys  contain- 
ing 2.50  per  cent  or  more  of  combined  carbon  and  a  sufficient  amount  of  silicon  the 
conversion  of  cementite  into  iron  and  graphite  takes  place  readily  and  can  be  carried 
to  completion,  combined  carbon  disappearing  altogether.  The  influence  of  tempera- 
ture and  time  upon  the  dissociation  of  cementite  was  to  be  expected.  Evidences  will 
be  presented  in  Chapter  XXVI  to  show  that  the  higher  the  temperature  at  which 
cementite  forms  the  more  readily  is  it  converted  into  iron  and  graphitic  carbon,  dur- 
ing solidification  and  subsequent  cooling.  The  influence  of  silicon  in  graphitizing 
cementite  could  likewise  have  been  anticipated  because  of  its  well-known  power  to 
cause  the  formation  of  graphitic  carbon  during  and  below  solidification.  It  will  be 
argued  in  Chapter  XXVI  that  the  formation  of  graphite  during  solidification  always 
results  from  the  dissociation  of  cementite,  that  is,  that  cementite  (Fe3C)  always  forms 
first  but  being  very  unstable  readily  breaks  up  into  iron  and  graphite,  its  dissociation 
being  promoted  (1)  by  slow  cooling  and  (2)  by  the  presence  of  silicon. 

The  conversion  of  combined  carbon  into  graphitic  or  rather  temper  carl  ion  finds 
an  important  industrial  application  in  the  manufacture  of  so-called  malleable  cast- 
iron  castings,  also  termed  "malleable  castings,"  "malleable  cast  iron,"  and  even 
"malleable"  pronounced  "mallable." 

The  metallography  of  these  castings  should  now  be  considered. 

Malleable  Cast-Iron  Castings.  —  The  adjective  "malleable"  always  used  in  de- 
scribing these  castings  is  misleading  for  it  suggests  a  malleability,  and  other  proper- 

398 


CHAPTER   XXIV  —  MALLEABLE   CAST   IRON  399 

ties  akin  to  those  of  steels,  which  malleable  castings  are  far  from  possessing.  By 
malleability  the  metallurgist  always  understands  that  property  which  makes  it  pos- 
sible to  convert  a  metallic  mass  into  commercial  shapes  by  rolling,  hammering,  etc., 
and  such  degree  of  malleability  is  not  present  in  malleable  cast  iron.  It  is  why  in  the 
author's  opinion  this  product  should  continue  to  be  classified  as  cast  iron  in  spite  of 
the  arguments  recently  presented  to  classify  it  as  steel  on  the  ground  that  it  is  more 
malleable  than  ordinary  cast  iron.  The  manufacture  of  malleable  cast-iron  castings 
consists  in  subjecting  to  a  long  annealing  treatment  cast-iron  castings  of  suitable 
composition  whereby  some  of  the  carbon  may  be  expelled  by  oxidation  while  most 
if  not  the  whole  of  the  remaining  is  converted  into  graphite  (temper  carbon).  The 
various  factors  influencing  this  operation  will  be  briefly  described. 

Original  Castings.  —  The  castings  to  be  subjected  to  the  malleablizing  treatment 
are  often  called  "hard"  castings  because  they  are  invariably  made  of  white  cast  iron 
and  are  therefore  very  hard  and  brittle.  The  reason  why  the  original  castings  must 
be  made  of  white  cast  iron  will  be  obvious  if  it  be  considered  that  the  malleability  to 
be  imparted  to  these  castings  is  to  result  chiefly,  and  in  some  cases  altogether,  from  a 
conversion  of  combined  carbon  into  temper  carbon  as  explained  above.  Any  graphite 
particle  existing  in  the  casting  will  be  unaffected  by  the  annealing  treatment  and  will 
be  a  source  of  weakness  in  the  finished  casting.  Clearly,  therefore,  the  hard  castings 
should  be  free  from  graphitic  carbon.  As  to  the  amount  of  carbon  that  should  be 
present,  theoretical  considerations  lead  us  to  conclude  that  the  less  carbon  the  more 
ductile  should  be  the  castings  after  thorough  malleablizing,  for,  after  all,  the  particles 
of  temper  carbon  while  much  less  injurious  than  the  hard  brittle  cementite  from 
which  they  are  derived  are  nevertheless  a  source  of  weakness  chiefly  because  of  their 
breaking  up  the  continuity  of  the  metallic  mass.  Again  a  smaller  proportion  of  car- 
bon necessarily  means  a  shorter  annealing  operation.  The  desirability  of  low  carbon 
content  in  cast-iron  castings  to  be  malleablized  is  universally  recognized  and  it  is 
partly  why  a  large  proportion  of  these  castings  are  made  in  the  "air"  furnace  or  open 
hearth  furnace,  as  by  their  use  the  percentage  of  carbon  can  be  lowered.  On  the 
other  hand,  as  already  mentioned,  if  the  proportion  of  combined  carbon  be  small  the 
graphitizing  takes  place  with  greater  difficulty.  These  considerations  point  to  the 
existence  of  a  lower  as  well  as  of  an  upper  limit  for  the  carbon  content  of  hard 
castings.  In  the  majority  of  cases  the  percentage  of  carbon  varies  between  2.50 
and  3.00  per  cent.  According  to  Moldenke  it  should  not  be  lower  than  2.75  per 
cent. 

The  beneficial  action  of  silicon  has  been  referred  to;  it  greatly  promotes  the  graphi- 
tizing  of  the  cementite.  It  might  seem  then  as  if  the  more  silicon  in  the  hard  casting 
the  better  at  least  up  to  some  2  or  3  per  cent.  Castings  of  white  cast  iron  cannot  be 
made,  however,  in  the  presence  of  much  silicon  since  this  element  would  cause  the 
formation  of  graphitic  carbon  on  solidification,  even  during  quick  cooling,  and  the 
casting  would  not  be  white.  And  since  large  castings  will  solidify  more  slowly  than 
smaller  ones  it  is  evident  that  in  large  castings  especially  but  a  relatively  small  amount 
of  silicon  can  be  allowed.  It  is  probably  true  that  as  much  silicon  should  be  intro- 
duced in  the  cast  iron  as  will  permit  the  making  of  white  castings  and  this  proportion 
of  silicon  will  necessarily  decrease  as  the  size  of  the  casting  increases.  In  practise  it 
is  found  to  vary  between  0.30  per  cent  in  large  castings  (4  inches  thick  or  more) 
and  1.25  per  cent  in  very  small  castings  for  like  solidification  conditions  and  subse- 
quent cooling.  In  the  majority  of  cases  the  silicon  content  varies  between  0.50  and 


400  CHAPTER   XXIV  —  MALLEABLE   CAST   IRON 

1  per  cent.1  Moldenke  writes  that  the  best  all  around  percentage  of  silicon  for  gen- 
eral castings  is  0.65.  A.  Lissner  recommends  0.9  per  cent  silicon  for  castings  one 
inch  thick. 

In  Moldenke's  opinion  the  manganese  should  not  exceed  0.60  per  cent  as  a  larger 
amount  is  liable  to  cause  trouble  on  annealing  and  difficulty  in  machining.  Phos- 
phorus according  to  the  same  authority  should  not  exceed  0.225  per  cent  while  sulphur 
should  not  exceed  0.07  and  preferably  not  0.05  per  cent.  These  figures  represent 
American  practise  only,  in  Europe  cast-iron  castings  being  malleablized  containing 
considerably  more  sulphur  and  phosphorus. 


Fig.  379.  —  Hard  casting  (white  cast  iron).     Magnified 
67  diameters.     (Boylston.) 

The  structure  of  a  hard  casting  of  suitable  composition  for  the  malleablizing 
process  is  shown  in  Figure  379.  It  will  be  seen  to  be  characteristic  of  the  structure 
of  white  cast  iron. 

Annealing  Operation.  —  The  hard  castings  are  placed  in  an  annealing  box  and 
firmly  packed  in  a  suitable  material.  The  boxes  are  then  placed  in  the  annealing 
furnaces  and  with  their  contents  exposed  to  a  desirable  temperature  for  a  suitable 
length  of  time.  These  various  steps  should  be  briefly  considered. 

Packing  Materials.  —  The  packing  material  affords  a  support  for  the  castings 
while  it  may  or  may  not  play  an  important  part  in  the  process  itself  according  to  its 
being  (1)  oxidizing  or  (2)  non-oxidizing.  In  his  pioneer  work,  in  1722,  Reaumur 
surrounded  white  cast  iron  with  crushed  iron  oxide,  the  oxygen  of  which  removed  a 
large  proportion  of  the  carbon  during  the  subsequent  long  annealing  treatment  at  a 
high  temperature.  Given  a  sufficiently  high  temperature  and  sufficiently  long  expo- 
sure nearly  the  whole  of  the  carbon  could  in  this  way  be  eliminated  from  small  cast- 

1  According  to  A.  Lissner  doubling  the  sulphur  reduces  the  speed  of  dissociation  of  the  cementite 
5  times  while  an  increase  of  0.8  per  cent  silicon  doubles  the  speed.  The  restraining  effect  of  0.05 
per  cent  sulphur  is  neutralized  by  an  increase  of  0.28  per  cent  silicon. 


CHAPTER   XXIV  —  MALLEABLE   CAST   IRON  401 

ings,  a  small  amount  of  it  only  remaining  as  temper  carbon.  It  will  be  evident  that 
in  the  operation  conducted  in  this  way  the  oxidizing  action  of  the  packing  is  of  more 
importance  in  producing  malleability  than  the  graphitizing  of  the  cementite.  The 
elimination  of  the  carbon  takes  place  through  the  well-known  reaction  between  the 
iron  oxide  (from  the  packing)  and  the  carbon  in  the  steel, 

Fe3C  +  O  =  3Fe  +  CO 

the  cementite  in  the  center  migrating  towards  the  outside  and  being  in  turn  acted 
upon  by  the  oxygen  of  the  packing  material. 

It  will  be  evident  that  this  elimination  must  necessarily  proceed  from  the  outside 
to  the  center,  so  that  if  one  should  interrupt  the  operation  after  a  relatively  short 
time  he  would  find  an  increasing  proportion  of  carbon  from  shell  to  center.  The  use 
of  an  oxidizing  and  therefore  chemically  active  packing  material  was  for  a  long  time 
considered  essential  in  the  malleablizing  process.  Later  investigations,  however, 
showed  that  white  cast  iron  could  be  made  malleable  solely  through  the  conversion 
of  combined  into  graphitic  carbon  with  very  little  if  any  removal  of  the  carbon  and 
that  an  oxidizing  packing  was  not  therefore  necessary.  Inert  packing  such  as  sand 
and  clay  may  be  used  and  satisfactory  malleable  castings  produced.  It  is  customary, 
however,  in  American  practise,  to  use  oxidizing  packings  but  to  depend  chiefly  on  the 
graphitizing  of  cementite  brought  about  by  annealing  for  the  desired  malleability  and 
strength.  Although  realizing  that  an  oxidizing  substance  as  a  packing  material  is 
not  indispensable  it  is  still  preferred  because  of  the  apparent  greater  strength  its  use 
confers  to  the  castings.  The  substances  most  used  are  powdered  iron  oxides  in  the 
shape  of  iron  ore,  mill  scale,  puddle  cinder,  "bull-dog,"  etc.,  and,  according  to  Mol- 
denke,  the  crushed  flakes  detached  from  the  annealing  pots  themselves. 

Annealing  for  Malleablizing.  —  The  malleability  imparted  to  white  cast  iron  by 
annealing  results  from,  as  explained  in  the  foregoing  pages,  (1)  the  hard  brittle  cemen- 
tite being  converted  into  minute  rounded  particles  of  soft  graphitic  (temper)  carbon 
and  (2)  the  burning  of  some  carbon  by  the  oxygen  of  the  packing  materials.  If  we 
depend  chiefly  upon  the  burning  of  the  carbon  for  the  desired  malleability,  so-called 
"white  heart"  castings  are  produced,  while  if  the  graphitizing  of  cementite  is  mainly 
sought,  so-called  "black  heart"  castings  are  obtained.  The  temperature  and  length 
of  the  annealing  operation  vary  according  to  the  kind  of  castings  wanted. 

The  annealing  operation  is  always  conducted  above  the  critical  point  Ac3.2.i  of 
the  white  cast  iron  and  of  course  below  its  solidification,  that  is  when  the  metal  exists 
as  an  aggregate  of  cementite  and  solid  solution  (saturated  austenite)  as  explained  in 
Chapter  XXVI.  It  is  not  known  whether  the  free  or  the  dissolved  cementite  is  graph- 
itized  first  or  whether  the  dissociation  of  both  kinds  proceeds  simultaneously.  Not- 
ing, however,  that  the  graphitizing  takes  place  more  readily  in  the  presence  of  a  con- 
siderable quantity  of  combined  carbon  and,  therefore,  of  free  cementite,  whereas  in 
the  presence  of  but  little  free  cementite  (cast  iron  with  less  than  2  per  cent  carbon)  it 
is  at  best  very  slow  and  remains  partial,  it  seems  logical  to  infer  that  it  is  the  free 
cementite  which  is  first  decomposed,  indeed  that  its  presence  is  necessary  to  cause 
the  graphitizing  of  the  dissolved  cementite. 

Annealing  for  "White  Heart"  Castings.  —  In  the  making  of  white  heart  castings 
a  very  large  proportion  of  the  carbon  is  removed  by  oxidation  through  the  use  (1)  of 
an  oxidizing  packing,  (2)  of  a  high  annealing  temperature,  and  (3)  of  a  long  annealing. 
The  original  malleable  castings  made  by  Reaumur  were  of  this  type  and  his  method 


402  CHAPTER   XXIV  —  MALLEABLE   CAST   IRON 

is  still  the  prevailing  one  in  Europe.  The  castings  are  annealed  for  four  or  five  days 
at  a  temperature  of  800  to  900  deg.  C.  By  removing  some  castings  from  the  anneal- 
ing box  from  time  to  time  and  examining  their  structure  the  progress  of  the  operation 
may  be  readily  observed.  The  following  transformations  are  noted  as  the  operation 
progresses:  (1)  a  narrow  white  rim  of  decarburized  metal  caused  by  the  burning  of 
the  carbon  from  the  outside  of  the  casting  and  a  dark  center  caused  by  the  graph- 
itizing  of  some  of  the  cementite,  (2)  a  broader  white  rim  and  a  smaller  and  darker 
core  due  to  the  burning  of  more  carbon  and  to  the  formation  in  the  center  of  the  cast- 
ing of  more  graphitic  carbon,  the  graphitizing  of  the  cementite  being  now  indeed 
possibly  complete,  (3)  the  fracture  of  the  metal  is  now  white  and  steely  to  the  very 
center  showing  that  most  of  the  carbon  has  been  removed  by  oxidation.  These  con- 
clusions are  fully  confirmed  by  the  microscopical  examination  of  the  structure  of 
castings  subjected  to  this  annealing  treatment  for  various  lengths  of  time.  As  this 
oxidation  of  the  carbon  is  necessarily  a  very  slow  process,  white  heart  castings  of 
small  size  only  are  made,  generally  not  over  J^  inch  thick.  For  malleablizing  larger 
castings  the  "black  heart"  process  soon  to  be  described  is  decidedly  superior. 

White  heart  castings  have  a  rather  coarse  fracture  and  structure  because  of  the 
high  and  prolonged  heating  to  which  they  were  exposed.  Since  they  resemble  low 
carbon  steels  in  composition  and  structure  it  would  seem  as  if  their  properties  could 
be  very  materially  improved  by  suitable  heat  treatment  as,  for  instance,  by  annealing 
followed  by  cooling  in  oil  or  in  air  according  to  their  carbon  content.  Moldenke 
states  that  the  European  white  heart  malleable  castings  bend  excellently  and  are 
very  good,  though  slightly  weaker  than  black  heart  castings. 

Annealing  for  "Black  Heart"  Castings.  —  For  the  production  of  black  heart  cast- 
ings, since  the  oxidation  of  carbon  is  of  secondary  importance,  the  annealing  tem- 
perature need  not  be  so  high  nor  indeed  is  it  necessary  to  use  an  oxidizing  packing. 
As  already  mentioned,  oxidizing  packings  are  generally  used,  however,  because  of  the 
apparent  greater  strength  they  impart  to  the  castings.  A  certain  amount  of  carbon, 
therefore,  is  burnt  at  the  surface  of  the  casting  causing  a  narrow  white  rim  while  the 
core  is  very  dark,  hence  the  name  of  black  heart  given  to  these  castings.  This  dark- 
ness of  the  core  is  due  as  we  now  understand  it  to  the  transformation  of  cement  car- 
bon into  temper  carbon. 

The  annealing  temperature  for  the  production  of  black  heart  castings  is  generally 
between  700  and  800  deg.  C.  in  the  case  of  air  furnace  iron,  that  is,  but  slightly  above 
the  critical  range  of  the  metal  and  the  length  of  time  at  the  full  converting  tempera- 
ture between  2J^  and  3  days.  Moldenke  states  that  cupola  iron  castings  should  be 
annealed  at  temperatures  some  65  to  120  deg.  higher  and  writes:1 

"Just  why  there  should  be  a  difference  in  the  temperature  required  for  castings 
of  the  same  composition  when  made  in  the  cupola  or  in  the  air  furnace,  is  one  of  the 
unsolved  problems.  It  may  be  chemical  in  that  the  degree  of  oxidation  has  its  effect 
on  the  opening  up  of  the  structure  under  the  influence  of  heat.  It  may,  on  the  other 
hand,  be  a  matter  of  molecular  physics,  and  depend  on  the  constitution  and  structure 
of  the  castings  as  made,  either  in  the  contact  of  fuel  with  iron,  or  not.  Possibly  it 
may  be  a  combination  of  both  the  chemical  and  the  physical.  Yet  the  problem  still 
remains  to  be  solved." 

In  completely  malleablized  black  heart  castings  practically  the  whole  of  the  car- 

1  A.  Lissner  writes  that  in  cupola  metal  containing  0.25  per  cent  sulphur  temper  carbon  begins  to 
form  at  760  deg.  C. 


CHAPTER   XXIV  —  MALLEABLE   CAST   IKON 


403 


bon  should  be  present  in  the  graphitic  condition,  the  castings  consisting  then  of  a 
narrow  case  of  decarburized  iron  and  of  a  core  made  up  of  ferrite  and  particles  of 
temper  carbon,  as  shown  in  Figure  380.  The  structure  of  the  core  both  before  and 
after  etching  is  further  illustrated  in  Figures  381  and  382.  If  the  malleablizing  is  but 
partial  because  of  too  low  a  temperature,  too  short  a  treatment,  too  little  silicon,  or 
for  some  other  reason,  considerable  dissolved  carbon  may  remain  which  in  cooling 
through  the  critical  range  will  give  rise  to  the  formation  of  pearlite.  The  structure  of 


Fig.  380.  —  Bliick   heart   malleable  casting  showing  decarburized 
rim.     Magnified   100  diameters.     (Boylston.) 


such  partially  malleablized  cast  iron  is  illustrated  in  Figure  383.  It  will  be  seen  to 
consist  of  graphitic  carbon,  pearlite,  and  ferrite.  The  mechanism  of  the  formation 
of  such  structures  is  obvious.  At  the  end  of  the  annealing  treatment  the  metal 
consisted  of  a  mass  of  austenite  in  which  were  embedded  a  number  of  particles  of 
temper  carbon;  on  slow  cooling  through  the  range  the  hypo-eutectoid  austenite 
rejected  some  free  ferrite  until,  its  composition  reaching  the  eutectoid  carbon  ratio, 
it  was  converted  into  pearlite.  It  will  be  noted  that  the  pearlitic  areas  which  should 
indicate  the  location  of  the  residual  austenite  are  situated  away  from  the  temper 
carbon  particles,  the  latter  being  surrounded  by  ferrite. 


404 


CHAPTER   XXIV  —  MALLEABLE   CAST    IRON 


• 


'«'** 


;> 


v» 

,     1 


Fig.  381.  —  Black  heart  casting.  Magnified  100  diameters.   Xot  etched. 
(F.  C.  Langonberg  in  the  author's  laboratory.) 


Fig.  382. —  Black  heart  malleable  casting.  Magnified  100  diameters.  Total  carbon 
2.64  per  cent.  Silicon  0.76  per  cent.  Phosphorus  0.137  per  cent.  Sulphiir  0.031 
per  cent.  Manganese  0.380  per  cent.  (Boylston.) 


CHAPTER   XXIV  —  MALLEABLE   CAST   IRON 


405 


The  annealing  of  white  cast  iron  may  he  so  incomplete  as  to  retain  so  much  dis- 
solved carhon  that  in  slow  cooling  free  cementite  as  well  as  pearlite  will  be  formed. 
In  this  case  the  austenite  which  existed  above  the  range  at  the  end  of  the  annealing 


1 


Fig.  383.  —  Partially  malleablized  cast  iron.  Magnified  100  diameters. 
Total  carbon  2.55  per  cent.  Silicon  0.77  per  cent.  Phosphorus  0.164 
per  cent.  (F.  H.  Franklin  of  Saunders  and  Franklin.) 


Fig.    384.  —  Partially   malleablized   cast 
iron.   Magnified  500  diameters.   (Wiist.) 

operation  was  hyper-eutectoid,  that  is,  it  contained  more  than  0.85  per  cent,  or  there- 
about, of  dissolved  carbon  and  on  cooling  through  the  range,  therefore,  liberated 
some  free  cementite.  It  is  evident  that  partially  malleablized  cast  iron,  that  is,  cast 


406  CHAPTER   XXIV  — MALLEABLE   CAST   IRON 

iron  still  containing  considerable  combined  carbon,  cannot  be  as  malleable  as  mallea- 
blized  cast  iron  free  from  combined  carbon  since  its  metallic  matrix  is  necessarily 
less  malleable. 

Cooling  from  Annealing  Temperature.  —  According  to  B.  L.  Leasman,  the  cool- 
ing from  the  annealing  temperature  to  1250  deg.  F.  should  be  very  slow  (42  hours) 
while  below  it  may  be  rapid. 

Gray  Cast  Iron  vs.  Malleable  Cast  Iron.  —  From  the  foregoing  description  of  the 
nature  and  manufacture  of  malleable  cast-iron  castings  it  is  evident  that  gray  cast 
iron  and  malleable  cast  iron  may  have  exactly  the  same  chemical  composition,  al- 
though, of  course,  the  former  will  generally  contain  more  silicon  and  total  carbon.  In 
spite  of  identical  or  nearly  identical  composition,  however,  these  two  metals  differ 
enormously  in  physical  properties,  gray  cast  iron  being  weak  and  brittle,  malleable 
cast  iron  much  stronger,  and  endowed  with  remarkable  shock-resisting  qualities.  To 
account  for  this  we  must  look  into  the  microstructure  of  both  metals  when  it  will  be 
observed  that  in  gray  cast  iron  the  graphite  occurs  in  large,  generally  curved,  flakes 
and  plates  whereas  in  malleable  cast  iron  it  is  present  in  small  rounded  particles,  and 
it  may  well  be  conceived  that  the  mode  of  occurrence  of  graphite  in  gray  iron  breaks 
up  the  continuity  of  the  metallic  matrix  much  more  effectively,  therefore,  weakening 
it  and  destroying  its  ductility.  Were  it  possible  during  the  solidification  of  cast  iron 
to  cause  the  graphite  to  occur  as  it  does  in  malleable  cast  iron,  there  is  no  reason  to 
doubt  but  that  it  would  be  as  strong  and  as  ductile. 


CHAPTER   XXV 

CONSTITUTION  OF  METALLIC  ALLOYS 

For  many  years  vague  and  conflicting  opinions  were  entertained  in  regard  to  the 
nature  of  metallic  alloys.  It  was  not  known  whether  these  intimate  associations  of 
two  or  more  metals  were  merely  mechanical  mixtures  or  chemical  compounds  while 
the  existence  of  solid  solutions  was  unsuspected.  The  application  to  the  study  of 
metallic  alloys  of  the  determination  of  the  solubility  curves  which  had  proved  so 
fruitful  in  investigating  the  mechanism  of  the  solidification  of  ordinary  (liquid)  solu- 
tions and  of  mixtures  of  melted  salts,  soon  followed  by  the  microscopical  examination 
of  their  structure  have  at  last  -revealed  their  true  constitution.  We  know  now  that 
metallic  alloys  may  be  considered  as  solution  of  high  freezing  (solidification)  point 
and,  therefore,  solid  at  ordinary  temperature,  whereas  the  old  conception  of  solution 
applied  only  to  substances  liquid  at  that  temperature.  It  is  evident,  however,  that 
the  location  of  the  freezing-point  of  a  substance  in  the  temperature  scale  can  have 
no  bearing  whatever  upon  its  constitution,  that  is,  upon  the  mode  of  occurrence 
of  its  constituents  and  the  nature  of  the  bond  uniting  them. 

In  these  chapters  the  constitution  of  metallic  alloys  will  be  considered  only  so  far 
as  necessary  to  understand  the  equilibrium  diagram  described  in  Chapter  XXVI  in 
which  steel  and  cast  iron  are  considered  as  alloys  of  iron  and  carbon,  i.e.  as  solutions 
of  these  elements,  liquid  at  a  very  high  temperature,  frozen  at  ordinary  temperature. 

Since  moreover  steel  and  cast  iron  are  considered  as  pure  iron-carbon  alloys  it  will 
suffice  for  our  purpose  to  deal  only  with  alloys  of  two  metals,  that  is,  with  binary 
alloys. 

The  constitution  of  alloys  is  revealed  chiefly  (1)  through  the  mechanism  of  their 
solidification  as  disclosed  by  their  "fusibility"  curves,  and  (2)  through  the  microscopi- 
cal examination  of  their  structure  after  solidification. 

Solidification  of  Pure  Metals.  —  Let  us  first  consider  the  solidification  of  a  pure 
metal  by  observing  its  rate  of  cooling  from  the  molten  to  the  solid  condition.  This 
involves  the  use  of  a  pyrometer,  preferably  a  thermo-electric  (Le  Chatelier)  instru- 
ment, the  hot  junction  of  which,  suitably  protected,  is  embedded  in  the  cooling  metal. 
By  recording  the  successive  intervals  of  time  in  seconds  required  for  each  successive 
cooling  through  ranges  of  temperature  say  of  10  deg.  C.,  and  plotting  time  against 
temperature  a  cooling  curve  of  the  type  shown  in  Fig.  385  is  obtained.1  The  teach- 
ings of  such  curves  are  obvious.  Starting  with  the  molten  metal  at  A ,  its  temperature 
being  T,  its  cooling  from  A  to  B,  while  its  temperature  is  falling  from  T  to  Ts  is  uni- 
formly retarded.  This  results  in  the  smooth,  nearly  straight  portion  AB  of  the  curve. 
The  cooling  of  a  molten  metal  above  its  solidification  point  is  in  this  respect  similar 
to  the  cooling  of  any  substance  free  from  thermal  critical  points;  the  cooling  curves 

1  Self-recording  pyrometers  may  also  bo  used. 
407 


408 


CHAPTER   XXV  —  CONSTITUTION   OF   METALLIC   ALLOYS 


obtained  in  such  cases  are  always  smooth  curves  indicating  a  uniform  increase  in 
time  as  the  temperature  of  the  substance  falls.  The  curve  of  Figure  1  indicates  the 
occurrence  of  a  sharp  critical  point  at  the  temperature  Ts  corresponding  to  the  hori- 
zontal portion  BC  of  the  curve.  It  is  evident  that  on  reaching  Ts  the  temperature 
of  the  metal  suddenly  ceased  to  fall  and  remained  stationary  during  an  interval  of 
time  represented  by  U'.  The  metal  then  resumed  a  normal  rate  of  cooling  which  was 
continued  to  atmospheric  temperature  as  indicated  by  the  smooth  portion  CD  which, 
were  it  not  for  the  jog  BC;  would  be  a  continuation  of  AB.  We  naturally  connect  this 
sudden  appearance  of  a  critical  point  in  the  cooling  curve  with  the  solidification  of  the 


r 


<b 


^.t     t' 

lime 

Fig.  385.  —  Typical  cooling  curve  of  pure  metal. 

metal.  It  clearly  indicates  (1)  that  the  metal  begins  to  solidify  at  the  temperature 
Ts,  (2)  that  while  it  is  solidifying  its  temperature  remains  constant,  (3)  that  the 
solidification  lasts  t'  - 1  seconds.  It  is  evident  that  during  its  solidification  the  metal 
was  exposed  to  the  same  cooling  influences  as  those  prevailing  above  and  below  it,  and 
since  its  temperature  nevertheless  remains  constant  it  must  be  that  heat  is  here 
liberated  in  amount  exactly  sufficient  to  make  up  for  the  heat  lost  by  radiation  and 
conductivity.  Any  attempt  at  increasing  the  rate  of  cooling  during  solidification  would 
result  in  hastening  solidification  and  not  in  lowering  the  temperature  of  the  metal. 
The  heat  evolved  during  the  solidification  of  a  substance  is  known  as  its  "latent  heat 
of  solidification."  In  Fig.  385,  AB  then  represents  the  cooling  of  the  liquid  metal, 
BC  its  solidification,  and  CD  the  cooling  of  the  solidified  metal.  On  heating  a  pure 
metal  from  below  to  above  its  melting-point,  a  similar  curve  is  obtained  indicating 
(1)  that  the  melting-point  coincides  with  the  solidification  point,  at  least  under  normal 


CHAPTER   XXV  —  CONSTITUTION   OF   METALLIC   ALLOYS 


409 


conditions,  and  (2)  that  the  melting  takes  place  at  a  constant  temperature,  being, 
therefore,  accompanied  by  an  absorption  of  heat. 

In  Fig.  386  the  cooling  curves  of  several  pure  metals  are  shown.  They  differ  only 
in  regard  to  the  location  of  the  horizontal  portions  indicating  the  temperatures  of 
solidification. 

The  structure  of  pure  metals  has  been  described  in  Chapter  IV,  when  they  were 
shown  to  be  made  up  of  polyhedral  crystalline  grains,  each  grain  consisting  of  true 
crystals  of  uniform  orientation.  As  an  instance  of  the  structure  of  pure  metals,  the 
structure  of  pure  lead  is  shown  in  Figure  387. 

S.OOO 


Qj  1000 

^ 

<b 


£ 
£ 


7~  /  rr->  e 

Fig.  386.  —  Cooling  curves  of  various  pure  metals. 


Solidification  of  Binary  Alloys  the  Constituents  of  which  Form  Solid  Solutions.  - 

The  cooling  or  solidification  curves  of  alloys  of  two  or  more  metals  may  be  constructed 
exactly  like  the  cooling  curve  of  a  pure  metal,  namely,  by  observing  the  rate  of  cooling 
as  the  temperature  is  lowered  from  above  the  melting-point  to  atmospheric  temperature 
and  plotting  the  intervals  of  time  against  the  corresponding  temperature  falls.  A 
number  of  binary  alloys  are  then  found  to  yield  cooling  curves  of  the  type  shown  in 
Fig.  387.  From  A  to  B,  that  is,  as  the  alloy  cools  from  T  to  Tb,  the  curve  is  smooth 
and,  therefore,  indicative  of  normal  cooling.  At  B  there  is  a  sudden  change  of  direction 
and  from  B  to  C,  that  is,  from  the  temperature  Tb  to  the  temperature  Tc,  the  cooling  of 
the  alloy  is  evidently  abnormally  slow.  From  C  to  D,  that  is,  from  the  temperature 
Tc  to  atmospheric  temperature,  the  cooling  is  again  normal.  Since  the  portion  BC 
of  the  curve  clearly  indicates  spontaneous  evolutions  of  heat  causing  a  marked  retarda- 
tion in  the  cooling  of  the  alloy  and  lasting  t'-t  seconds,  we  naturally  infer  that  it  cor- 
responds to  its  solidification.  It  follows  from  the  appearance  of  the  cooling  curve 


410  CHAPTER  XXV  —  CONSTITUTION   OF   METALLIC   ALLOYS 

that  alloys  yielding  such  curves  do  not,  like  pure  metals,  solidify  at  a  constant  tempera- 
ture but  that  their  solidification,  on  the  contrary,  lasts  t'-t  seconds  while  their  tempera- 
ture falls  from  Tb  to  Tc.  Summing  up,  AB  indicates  the  cooling  of  the  molten  alloy, 
B  the  beginning,  and  C  the  end  of  its  solidification,  It'  the  time  required  for  its  solidifi- 
cation, Tb  Tc  the  fall  of  temperature  during  solidification,  and  CD  the  cooling  of  the 
solidified  alloy.  Above  the  point  B,  therefore,  the  alloy  is  entirely  liquid,  below  C  it 
is  entirely  solid,  while  between  B  and  C  it  is  partly  liquid  and  partly  solid.  The  point  B 
is  accordingly  called  the  liquidus  point  and  C  the  solidus  point. 

Binary  alloys  whose  cooling  curves  are  of  the  type  shown  in  Fig.  388  are  known 
to  be  solid  solutions.  In  these  alloys  the  component  metals  which  are  completely 
merged  in  the  liquid  condition  remain  likewise  so  completely  merged  after  solidifica- 
tion that  their  separate  existence  cannot  be  detected  by  microscopical  examination  or 


Fig.  387.  —  Pure  lead.    Magnified  20  diameters.     (F.  C. 
Langenbcrg  in  the  author's  laboratory.) 

other  physical  means.  They  formed,  on  solidifying,  homogeneous  crystals  containing 
both  metals  in  indefinite  proportions.  These  crystals  are  sometimes  called  "mixed 
crystals"  and  substances  yielding  them  " isomorphous "  mixtures  by  which  it  is  meant 
that  only  isomorphous  substances '  can  yield  mixed  crystals  or  in  other  words  can  form 
solid  solutions.  The  expression  "solid  solution"  is  much  preferable  and  is  now  quite 
universally  used,  at  least  by  the  English  and  French. 

The  mechanism  of  the  formation  of  solid  solutions  of  two  metals  should  be  ex- 
amined more  closely.  Let  us  assume  that  a  certain  proportion  of  the  metal  M  of 
relatively  low  melting-point  is  alloyed  with,  or  dissolved  in,  the  metal  M'  of  higher 
melting-point.  The  metal  M  may  be  considered  as  the  solute  and  M'  as  the  solvent. 
It  is  believed  that  when  solidification  begins  homogeneous  crystals  of  M  and  M'  are 
formed  but  that  they  contain  a  smaller  proportion  of  the  fusible  metal  M  than  the 

1  Isomorphous  substances  are  those  that  are  capable  of  crystallizing  in  the  same  crystallographic 
forms. 


CHAPTER   XXV  —  CONSTITUTION   OF   METALLIC   ALLOYS 


411 


liquid  bath,  which  is  thereby  enriched  in  .I/.'  On  further  cooling  these  crystals  grow 
but  the  crystalline  matter  now  deposited  contains  more  of  the  metal  M  than  the  crystals 
first  formed,  although  still  less  than  the  molten  bath  which  is  further  enriched  in  M 
and  so  on,  the  crystals  growing  through  successive  additions  of  crystalline  matter  con- 
taining increasing  proportions  of  the  dissolved  and  relatively  fusible  metal  M,  and 
approaching,  therefore,  although  not  reaching,  the  composition  of  the  molten  metal 
until  finally  the  last  drop  solidifies.  Meanwhile,  as  the  temperature  is  lowered  through 
and  below  the  solidification  range,  diffusion  takes  place  within  the  crystals  so  that 
finally  they  become  chemically  homogeneous  provided  time  be-given  (through  slow 


T 


-I 


<b 


5 


t 


t' 
7~/  m  e . 


Fig.  388.  —  Typical  cooling  curve  of  binary  alloy  whose  component  metals  form  a 

solid  solution. 

cooling)  for  complete  diffusion.     Like  pure  metals,  alloys  whose  component  metals 
form  solid  solutions,  are  composed  of  polyhedral  crystalline  grains  (see  Fig.  389). 

Fusibility  Curves  of  Binary  Alloys  whose  Component  Metals  are  Completely 
Soluble  in  each  Other  when  Solid.  —  So-called  "fusibility  curves"  or  "equilibrium 
diagrams"  are  obtained  from  any  series  of  alloys  by  constructing  the  solidification 
curves,  as  explained  above,  of  a  number  of  alloys  of  that  series  and  plotting  on  a  single 

1  It  is  because  the  bath  becomes  richer  in  the  more  fusible  metal  M  that  its  melting-point  is 
lowered,  resulting  in  its  solidification  covering  a  considerable  and  falling  range  of  temperature.  If 
the  crystals  first  formed  had  the  same  composition  as  the  bath,  solidification  would  take  place  at  a 
constant  temperature.  Witness  the  solidification  of  pure  metals,  of  eutectic  alloys,  and  of  chemical 
compounds:  the  solidifying  metal  having  the  same  composition  as  the  liquid  from  which  it  forms, 
solidification  takes  place  at  a  constant  temperature. 


412  CHAPTER   XXV  —  CONSTITUTION   OF   METALLIC   ALLOYS 

diagram  the  evolutions  of  heat  observed.  This  has  been  done  in  Fig.  390  by  uniting 
the  BB'B"  .  .  .  and  the  CC'C"  .  .  .  points  denoting  respectively  the  beginning  and  the 
end  of  the  solidification  of  a  number  of  alloys  A  A' A"  .  .  .  Leaving  out  the  independent 
cooling  curves  used  for  the  construction  of  the  fusibility  curve,  the  diagram  shown  in 
Fig.  391  is  obtained,  the  co-ordinates  being  now  temperature  and  composition.  The 
figure  is  typical  of  the  fusibility  curves  of  binary  alloys  whose  component  metals  are 
entirely  soluble  in  each  other  when  solid.  They  are  composed  of  two  branches,  one 
concave  the  other  convex,  uniting  the  melting-points  of  the  constituent  metals. 
MLM'  is  known  as  the  liquidus  because  any  alloy  of  the  series  above  that  line  is 
entirely  liquid,  while  MSM'  is  called  the  solidus  because  any  alloy  below  it  is  entirely 
solid.  Within  the  area  MLM'SM  the  alloys  are  partly  liquid  and  partly  solid.  The 
solidification  of  these  alloys  should  be  further  described  with  the  help  of  this  diagram. 
Let  us  assume  an  alloy  the  composition  of  which  is  represented  by  the  point  P  in  the 


Fig.  389.  —  Copper-zinc  alloy.  Copper  50 
per  cent.  Magnified  200  diameters.  Ho- 
mogeneous solid  solution.  (Guillet.) 

diagram  and  containing,  therefore,  75  per  cent  of  the  low  melting-point  metal  M  and 
25  per  cent  of  the  less  fusible  metal  M'.  An  alloy  of  that  composition  at  the  tempera- 
ture T  is  entirely  liquid  since  its  condition  is  represented  by  a  point  situated  above  the 
liquidus.  As  the  alloy  cools  from  P  to  I  its  temperature  falling  from  T  to  t,  it  still 
remains  entirely  liquid.  At  I,  temperature  t,  solidification  begins  through  the  forma- 
tion of  crystals  whose  composition  must  be  represented  by  the  point  s  at  the  inter- 
section of  the  solidus  and  of  a  horizontal  line  through  I,  for  it  is  evident  that  the  only 
crystals  that  can  be  in  equilibrium  at  the  temperature  t  with  the  liquid  of  composition 
I  must  have  the  composition  s.  To  clarify,  if  the  crystals  in  equilibrium  with  the 
liquid  I  at  the  temperature  t  have  not  the  composition  s,  then  they  must  necessarily 
contain  either  more  of  the  metal  M'  or  less  of  that  metal.  In  other  words,  their  com- 
position may  be  represented  by  the  point  x  to  the  left  of  s  or  by  the  point  y  to  its 
right.  It  is  evident  that  the  composition  of  the  crystals  forming  at  the  temperature  t 
from  a  liquid  of  composition  I  cannot  have  the  composition  x  since  the  corresponding 
point  falls  within  the  area  MLM'SM  and  since  any  point  in  this  area  cannot  represent 
the  composition  of  the  crystals  existing  at  the  corresponding  temperature  but  must 


CHAPTER  XXV  —  CONSTITUTION   OF   METALLIC   ALLOYS  413 

ff" 


Fig.  390.  —  Diagram  showing  how  fusibility  curves  are  constructed. 


M 

M' 


loo 
o 


75-  50 

2.S  SO 

Cofnp  os  if  ion . 


25 
75- 


O 
ICO 


Fig.  391.  —  Typical  fusibility  curve  of  binary  alloys  whose  component  metals  form  solid  solution. 

represent  on  the  contrary  the  average  composition  of  the  partly  solidified  alloy,  hence 
the  crystals  forming  at  t  in  the  case  we  are  considering  cannot  have  a  composition 
represented  by  a  point  to  the  left  of  s.  Assuming  the  composition  of  these  crystals  to 


414  CHAPTER   XXV  —  CONSTITUTION   OF   METALLIC   ALLOYS 

be  represented  by  the  point  y  to  the  right  of  s,  then  it  is  evident  that  these  crystals 
must  have  formed  at  z,  that  is,  at  a  temperature  considerably  higher  than  t,  which 
cannot  be  since  our  alloy  does  not  begin  to  solidify  before  the  temperature  t  is  reached. 
Hence  the  composition  of  the  crystals  forming  at  t,  when  the  alloy  P  begins  to  solidify, 
cannot  be  represented  by  a  point  to  the  right  of  s.  Clearly  the  point  s  must  indicate  the 
composition  of  those  crystals.  As  the  alloy  cools  from  I  to  o,  that  is,  from  t  to  t',  the 
crystals  which  began  to  form  at  I  continue  to  grow  by  gradual  deposition  of  crystalline 
matter  progressively  richer  in  M  while  the  remaining  liquid  bath  likewise  becomes 
richer  in  M  and  consequently  more  fusible,  its  varying  composition  being  represented 
by  II'.  For  any  point  o  within  the  area  MLM'SM,  that  is,  when  the  alloy  is  in  the  proc- 
ess of  solidification,  the  composition  of  the  crystals  in  equilibrium  at  the  correspond- 
ing temperature  t'  with  the  solution  I'  is  represented  by  the  point  s'.  This  must 


Fig.  392.  —  Iron-copper  alloy.  Copper  10  per  cent. 
Magnified  125  diameters.  Heterogeneous  crys- 
talline grains.  The  dark  parts  are  richer  in 
copper,  the  lighter  parts  richer  in  iron.  (Stead.) 

necessarily  mean  that  as  the  metal  cools  from  t  to  t'  and  while  the  crystals  are  grow- 
ing, diffusion  must  necessarily  take  place  in  each  crystal  so  that  the  concentric  layers 
of  varying  composition  of  which  we  may  conceive  that  they  are  initially  composed, 
assume  the  same  composition  s',  the  crystals  being  now  homogenous.  At  s",  tem- 
perature t",  the  solidification  is  complete.  The  last  drop  of  liquid  to  solidify  had  the 
composition  I".  By  diffusion  the  crystals,  which  began  to  form  at  s  and  grew  from  s  to 
s",  that  is,  as  the  temperature  fell  from  t  to  t",  have  assumed  a  homogenous  chemical 
composition.  While  this  diffusion,  however,  needed  to  produce  homogenous  crystals, 
readily  takes  place  during  the  crystallization  of  liquid  solutions,  the  case  is  different 
with  solid  solutions.  Unless  solidification  and  subsequent  cooling  have  been  sufficiently 
slow,  solid  solutions  may  remain  heterogenous,  i.e.  the  different  layers  of  crystalline 
matter  of  which  each  crystal  is  composed  may  not  be  of  identical  composition,  the 
proportion  of  the  most  fusible  metal  increasing  from  center  to  outside  (see  Fig.  392). 
As  an  instance  of  binary  alloys  forming  solid  solutions  the  fusibility  curve  of  gold- 
platinum  alloys  is  reproduced  in  Figure  393. 

Binary  Alloys  Forming  Definite  Compounds  and  Solid  Solutions.  —  When  two 


C'H, \PTKR  XXV  —  CONSTITUTION   OF   METALLIC   ALLOYS 


415 


metals  unite  to  form  solid  solutions,  it  does  not  necessarily  imply  that  they  may  not 
chemically  combine  with  each  other  as  well.  Important  instances  are  known  of  metals 
forming  a  definite  chemical  compound,  which  compound  is  then  dissolved  in  the  re- 
maining excess  metal.  Two  metals  M  and  M',  for  instance,  may  unite  chemically  to 
form  the  compound  MxM'y  and  unless  the  alloy  contains  the  two  metals  exactly  in 
the  required  atomic  proportions,  the  compound  may  form  a  solid  solution  with  the 
excess  of  metal  M  or  of  metal  M ',  as  the  case  may  be.  In  such  cases  the  alloys  should 
be  considered  not  as  alloys  of  two  metals  but  of  one  metal  and  of  one  chemical  com- 
pound of  two  metals,  when  the  mechanism  described  to  explain  the  solidification  of 
solid  solutions  will  be  found  applicable.  Indeed  we  may  conceive  the  existence  of 
alloys  of  two  metals  in  which  two  definite  compounds  are  formed,  mutually  soluble 
and,  therefore,  forming  solid  solutions.  Such  alloys  should  be  considered  not  as  alloys 


/coo 
noo 

1600 
JSOO 
If  00 
1300 

100 
/too 

1000 
600 

BOO 
•TOO 


20  4O  SO 


to 


Pt  (174-4-°) 


too  'APt- 
by  weight 


Au(1064-°) 


Solid*  solution- 
CAu,  +  Pt) 


O      to      20      3O      +O      so      eo      70     so 

Fig.  393.  —  Fusibility  curve  alloys  of  gold  and  platinum.     (Desch.) 


9O      IOO 

AtomKPt 


of  two  pure  metals  but  of  two  chemical  compounds,  the  mechanism  of  their  solidifica- 
tion being  then  identical  to  that  of  alloys  of  two  metals. 

Binary  Alloys  whose  Component  Metals  are  Insoluble  in  Each  Other  in  the  Solid 
State.  —  If  two  metals,  although  soluble  in  the  liquid  state,  are  insoluble  when  solid,  it 
is  evident  that  on  solidification  they  must  crystallize  separately  into  distinct  crystals 
readily  distinguishable  under  the  miscroscope,  in  other  words,  that  the  solid  alloy  must 
be  an  aggregate  of  the  two  metals.  The  study  of  the  mechanism  of  the  solidification 
of  such  alloys  and  of  their  microstructure  should  receive  some  attention. 

The  typical  cooling  or  solidification  curve  of  an  alloy  of  two  metals  insoluble  in 
each  other  when  solid  is  shown  in  Figure  10.  The  curves  are  obtained  as  previously 
explained  by  observing  the  time  required  by  the  alloy  to  cool  through  successive  and 
equal  ranges  of  temperature  and  by  plotting  the  times  against  the  corresponding  falls 
of  temperature,  or  more  conveniently  and  accurately  by  the  use  of  self-registering 
pyrometers.  The  curve  of  Fig.  394  will  be  seen  to  consist  of  four  parts,  namely,  AL 


416 


CHAPTER   XXV  —  CONSTITUTION   OF   METALLIC   ALLOYS 


indicating  a  normal  rate  of  cooling,  LL'  a  retarded  cooling,  L'S  a  stationary  tempera- 
ture, and  SB  a  normal  rate  of  cooling  to  atmospheric  temperature.  It  is  logical  to 
infer  that  AL  represents  the  uniform  cooling  of  the  liquid  alloy  and  SB  the  uneventful 
cooling  of  the  solid  metal,  L  being  the  liquidus  point,  and  S  the  solidus.  The  portion 
of  the  curve  LL'S  represents  the  solidification  of  the  alloy  as  the  temperature  falls 
from  t  to  t'.  It  will  be  observed  (1)  that  the  solidification  begins  at  L,  temperature  t, 
(2)  that  it  proceeds  from  L  to  L'  as  the  temperature  is  falling,  and  (3)  that  the  end  of 
the  solidification  takes  place  at  a  constant  temperature,  namely,  t',  as  indicated  by  the 


Fig.  394.  —  Typical  cooling  curve  of  binary  alloy  whose  component  metals  are 
insoluble  in  each  other  in  the  solid  state. 

horizontal  portion  L'S,  the  temperature  of  the  alloy  remaining  constant  during  T'-T 
seconds. 

In  Fig.  395  the  solidification  curves  of  a  number  of  alloys  of  the  same  series  have 
been  constructed  as  explained  above,  the  alloys  arbitrarily  selected  containing  respec- 
tively 10,  20,  40,  60,  and  80  per  cent  of  metal  M.  It  will  be  noted  that,  with  the 
exception  of  the  alloy  containing  40  per  cent  of  M ,  each  alloy  exhibits  two  evolutions 
of  heat,  namely  an  upper  evolution,  L,  at  varying  temperatures  and  a  lower,  E,  always 
at  the  same  temperature  regardless  of  the  composition  of  the  alloy.  By  uniting  the 
upper  points  and  the  lower  ones  as  indicated  by  dotted  lines  in  Figure  1 1 ,  the  so-called 
fusibility  curve  or  equilibrium  diagram  of  that  series  of  alloys  is  obtained.  In  Fig.  396 
the  fusibility  curve  only  is  represented,  the  independent  cooling  curves  used  for  its 


CHAPTER   XXV  —  CONSTITUTION   OF   METALLIC   ALLOYS 


417 


construction  having  been  omitted.     The  co-ordinates  are  now  composition  and  tem- 
perature.   The  curve  indicates  clearly  the  beginning  and  the  end  of  the  solidification 


60% 


3o% 


§ 

<U 

I 


M%    O 

M'%   /oo 


Fig.  :$!)">.  —  Diagram  showing  how  fusibility  curves  are  constructed. 


Co  mp  os  if  ion . 


Fig.  3()<>.  — Typical  fusibility  ctirve  of  binary  alloys  whose  component  metals  are  insoluble  in  each 

other  in  the  solid  state. 

of  any  alloy  of  the  series.  The  solidification  of  an  alloy  whose  composition  corresponds 
to  the  point  R,  for  instance,  and  which,  therefore,  contains  20  per  cent  of  metal  M  and 
80  per  cent  of  metal  M'  evidently  begins  at  N  and  ends  at  P.  The  fusibility  curve  is 


418  CHAPTER   XXV  —  CONSTITUTION   OF   METALLIC   ALLOYS 

made  up  of  three  branches,  namely,  the  two  intersecting  lines  LE  and  L'E  starting 
respectively  from  the  melting-points  of  the  two  constituent  metals  and  a  horizontal 
line  SS'  passing  by  the  point  of  intersection  E  of  the  first  two. .  This  is  the  typical 
fusibility  curve  of  binary  alloys  whose  component  metals  are  insoluble  in  each  other 
in  the  solid  condition.  The  solidification  of  these  alloys  should  now  be  examined  more 
closely.  Several  features  are  obvious.  The  different  alloys  begin  to  solidify  at  differ- 
ent temperatures  according  to  their  composition.  At  first,  as  the  percentage  of  .!/ 
increases  from  0  to  40  (a  proportio'n  arbitrarily  selected),  the  solidification  point  is 
lowered  from  L  to  E,  while  with  further  increase  of  M  from  40  to  100  per  cent,  the 
solidification  point  is  raised  from  E  to  L'.  The  solidification  of  all  alloys,  however, 
ends  at  the  same  temperature,  namely,  at  the  temperature  S.  Clearly,  LEU  is  the 
liquidus  and  SES'  the  solidus.  The  alloy  containing  40  per  cent  of  M  is  evidently  the 
most  fusible  alloy  of  the  series  since  it  remains  liquid  until  the  temperature  S  is  reached 
while  other  alloys  begin  to  solidify  at  higher  temperatures.  This  alloy  of  lowest 
melting-point  is  known  as  the  "eutectic"  alloy,  from  the  Greek  meaning  "well  melt- 
ing." It  is  evident  that,  like  pure  metals,  eutectic  alloys  solidify  at  a  constant  tempera- 
ture, namely  the  eutectic  temperature.  Many  aqueous  solutions  also  give  rise  to  the 
formation  of  solutions  of  lowest  freezing-points  called  "  cryohydrates  "  and  which  were 
at-  first  supposed  to  be  true  chemical  compounds,  that  is,  hydrates  containing  salt  and 
water  molecules  in  atomic  proportions.  And,  likewise,  eutectic  alloys  were  at  first 
supposed  to  be  definite  chemical  compounds  of  the  two  metals.  They  are  now  known 
to  be  aggregates  of  these  metals  generally  very  finely  divided.  This  will  be  made  clear 
by  following  the  solidifications  of  three  alloys,  R,  R' ,  and  R"  (Fig.  39(5).  The  alloy  R 
contains,  according  to  the  diagram,  20  per  cent  of  the  metal  .17  and  80  per  cent  of  the 
metal  M'.  Since  it  contains  less  of  the  metal  M  than  the  eutectic  alloy,  we  may  for 
convenience  refer  to  it  as  an  hypo-eutectic  alloy,  although  of  course  in  regard  to  the 
content  of  M'  it  would  be  hyper-eutectic.  In  cooling  from  R  to  N  the  alloy  remains 
liquid.  At  N  solidification  begins  through  the  formation  of  pure  crystals  of  M',  that 
is,  of  the  metal  which  is  present  in  excess  above  the  eutectic  ratio.  The  formation  of 
pure  crystals  of  M'  continues  as  the  alloy  cools  from  N  to  P  and  meanwhile  the  portion 
of  the  alloy  remaining  liquid  (we  may  call  it  the  mother  metal)  becomes  gradually 
richer  in  M,  i.e.  it  approaches  gradually  the  composition  of  the  eutectic  alloy.  Finally 
at  P,  temperature  S,  the  remaining  liquid  has  exactly  the  eutectic  composition  and 
now  solidifies  at  a  constant  temperature,  the  solidification  temperature  of  the  eutectic 
alloy.  In  other  words,  as  the  alloy  cools  from  N  to  P  with  formation  of  pure  crystals 
of  metal  M'  the  composition  of  the  portion  of  the  alloy  remaining  liquid  varies  ac- 
cording to  the  line  NKE,  reaching  the  composition  E,  that  is,  the  eutectic  composition, 
always  at  the  same  temperature  regardless  of  the  initial  composition  of  the  alloy. 
When  the  alloy  has  cooled  to  0,  for  instance,  a  point  between  LE  and  tiE,  it  is  partly 
liquid  and  partly  solid,  its  temperature  is  T  and  the  composition  of  the  liquid  portion 
is  represented  by  K,  that  is,  it  contains  30  per  cent  of  the  metal  M.  On  further  cooling 
from  0  to  P  the  composition  of  the  mother  metal  shifts  from  K  to  E. 

In  the  case  of  the  alloy  R",  containing  a  larger  proportion  of  the  metal  M  than  the 
eutectic  alloy  and  which  we  may,  therefore,  consider  to  be  hyper-eutectic,  when  it 
reaches  the  point  N'  its  solidification  begins,  pure  crystals  of  the  metal  M  being 
formed  while  the  molten  bath  becomes  gradually  richer  in  M'  gradually  approaching, 
therefore,  the  composition  of  the  eutectic  alloy,  until  at  the  temperature  S  that  com- 
position is  reached  when  the  remaining  liquid  solidifies  at  a  constant  temperature. 


CHAPTER   XXV  —  CONSTITUTION  OF   METALLIC   ALLOYS 


419 


Any  point  0'  situated  between  L'E  and  8'E  indicates  an  alloy  in  part  solid  and  in 
part  liquid;  its  temperature  is  7"  and  the  composition  of  the  liquid  is  represented  by 
K'.  On  cooling  from  0'  to  P'  additional  crystals  of  pure  M  are  formed,  or  those  already 
formed  continue  to  grow  while  the  composition  of  the  molten  metal  shifts  from.  K'  to  E. 

Starting  with  the  alloy  R'  of  eutectic  composition,  it  remains  liquid  until  at  E, 
temperature  S,  it  solidifies  at  a  constant  temperature,  there  being  no  excess  metal  to 
be  rejected. 

Seeing  that  the  branch  LE  corresponds  to  the  formation  of  pure  crystals  of  the 
metal  M'  and  the  branch  L'E  to  the  formation  of  pure  crystals  of-the  metal  M ,  the  con- 
clusion seems  irresistible  that  their  point  of  intersection  E  must  correspond  to  the 


Composition. 

l-'ifj;.  :-i'.l7.    -  Dmfjnun  depicting  the  mechanism  of  the  solidification  of  alloys  whose  component  metals 
are  insoluble  in  each  other  in  the  solid  state. 

simultaneous  formation  of  crystals  of  metal  M  and  of  metal  M'  and  that  the  eutectic 
alloy,  therefore,  must  be  a  finely  divided  aggregate  of  M  and  M'.  In  other  words,  at 
any  point  on  the  branch  LE,  the  alloy  is  saturated  with  the  metal  M'  so  that  the  lower- 
ing of  its  temperature  must  cause  the  separation  of  M'  crystals  with  corresponding 
lowering  of  the  saturation  point,  that  is,  of  the  solidification  point  of  the  bath.  In  a 
similar  way,  at  any  point  on  the  branch  L'E,  the  alloy  is  saturated  with  M  and  a  fall 
in  its  temperature  must  result  in  the  formation  of  M  crystals  while  the  solidification 
point  of  the  portion  remaining  liquid  is  thereby  lowered.  At  E  the  alloy  is  saturated 
with  both  metals  so  that  any  attempt  at  lowering  its  temperature  must  result  in  the 
simultaneous  deposition  of  crystals  of  M  and  of  M ' ,  and  since  the  composition  of  the 
bath  remains  the  same,  solidification  now  takes  place  at  a  constant  temperature. 
Hence  the  constitution  of  eutectic  alloys  and  the  reason  for  their  constant  freezing 
temperature. 

It  has  been  attempted  in  Figure  397,    to  depict  graphically  the  mechanism  of  the 


420 


CHAPTER  XXV  — CONSTITUTION   OF  METALLIC  ALLOYS 


solidification  of  hypo-eutectic,  eutectic,  and  hyper-eutectic  alloys.     As  diagrams  of  this 

kind  have  already  been  used  in  these  lessons  the  present  one  will  be  readily  understood. 

When  the  alloy  R,  for  instance,  reaches  the  point  A",  crystals  of  M'  form,  their 


I 

«5 

o 

I 

o 

1 

4 

M  % 
M'% 

Chemical    composition. 

.Fig.  398.  —  Diagram  showing  the  structural  composition  of  binaiy  alloys  whose  component  m-tals 
are  insoluble  in  each  other  in  the  solid  stale. 


75 
.SO. 

/ 

Metaf 
M' 

/^        ^^                      A 

I/^E  —fcufecfic-  X, 

fe/W 

A^ 

3O 
0 

£  c 

0                      2. 
OO                     £ 

]  "  "  

1  i  i  

O  -4Q  6O  <f 
IO  6O  -4O  <2 

1  -X 

0                        /( 

0 

Chemical    composition. 


Fig.  399.  —  Diagram  showing  the  fusibility  curve  and  the  structural, composition  of  binary  aliens 
whose  component  metals  are  insoluble  in  each  other  m  the  solid  state. 

formation  as  the  metal  cools  frotn  N  to  P  being  represented  by  the  area  NPP'.  At  P 
the  residual  liquid,  now  of  eutectic  composition,  solidifies  at  a  constant  temperature. 
At  any  point  0  the  composition  of  the  portion,  still  liquid  is  represented  by  A'.  The 
completely  solidified  metal  will  be  made  up  of  ab  per  cent  of  metal  M  and  be  per  cent 
of  eutectic  alloy. 


CIIAPTKR    XXV   -COXSTHTTIOX   OK   METALLIC   ALLOYS 


421 


It  will  not  be  necessary  to  describe  at  greater  length  the  graphical  representation 
of  the  solidification  of  the  alloys  R'  and  R". 

The  structural  composition  of  alloys  of  two  metals  eomplotelyinsoluble  in  each  other 
in  the  solid  condition  may  conveniently  be  represented  graphically  as  shown  in  Figure 

1 700 


326  ^ 


228 


Pb 


Sb 


632 

600 

500 
400 
300 
200 


10 


60 


90       100 


20        30         40         50         60         70 
Percentages  of  antimony  by  weight 

Fig.  4(X).  —  Fusibility  curve  of  alloys  of  lead  and  antimony.     (Roland-(!osselin.) 


Kig.  401.  —  Typical  structures  of  alloys  whose  component  metals 
are  insoluble  in  each  other  in  the  solid  state,  x,  excess  metal 
M'  and  eutectic;  ij,  eutectic;  z,  excess  metal  M  and  eutectic. 
(Gulliver.) 

398.  The  interpretation  of  this  diagram  is  obvious.  An  alloy  containing  20  per  cent 
of  the  metal  .17,  for  instance,  will  be  made  up  of  ab  per  cent  of  M'  and  be  per  cent  of  eu- 
tectic alloy.  With  40  per  cent  of  HI  the  alloy  is  wholly  of  eutectic  composition  while  with 
SO  per  cent  of  .!/,  for  instance,  it  contains  a'b'  of  M  crystals  and  b'c'  per  cent  of  eutectic. 


422 


CHAPTER   XXV  —  CONSTITUTION   OF   METALLIC   ALLOYS 


The  composition  diagram  may  with  advantage  be  combined  with  the  equilibrium 
diagram  as  it  has  been  done  in  Figure  399.  Taking  an  alloy  whose  composition  is 
represented  by  R,  for  instance  (20  per  cent,  M  and  80  per  cent  M'),  it  is  seen  (1)  that 
it  begins  to  solidify  at  N,  (2)  that  as  it  cools  from  N  to  P  pure  crystals  of  M'  are  formed, 
(3)  that  the  percentage  of  M'  thus  liberated  is  represented  by  PK,  (4)  that  at  P  the 
remaining  molten  alloy  now  of  eutectic  composition  solidifies,  and  (5)  that  KO  repre- 
sents the  percentage  of  this  eutectic  alloy. 

The  graphical  method  used  in  these  chapters  for  representing  the  structural  com- 
position of  alloys  was  first  suggested  by  the  author  in  1896.  It  has  since  been  widely 
used  and  Tammann  employs  it  to  represent  the  heat  liberated  by  the  solidification  of 
the  eutectic  or  the  time  taken  for  its  solidification,  it  being  evident  that  both  heat 
and  time  must  necessarily  be  proportional  to  the  amount  of  eutectic  alloy  formed.  In 


• 


^•sy 


Fig.  402.  —  Eutectic  alloy  of  bismuth  and  tin. 
Magnified  200  diameters.     (Desch.) 

Fig.  399,  therefore,  the  vertical  distances  of  the  shaded  area  are  proportional  to  the 
times  during  which  the  temperature  of  the  alloys  remained  constant  while  the  residual 
baths  of  eutectic  composition  were  solidifying.  The  fusibility  curve  of  lead-antimony 
alloys  is  reproduced  in  Fig.  400  as  an  instance  of  alloys  whose  component  metals  are 
entirely  insoluble  in  each  other  after  solidification. 

From  the  foregoing  it  appears  that  solidified  alloys  of  two  metals  insoluble  in  each 
other  are  aggregates  of  these  two  metals  and  that  three  types  of  structure  are  to  be 
expected  (1)  the  structure  of  hypo-eutectic  alloys  composed  of  crystals  or  crystalline 
grains  or  particles  of  one  metal  embedded  in,  or  surrounded  by,  some  eutectic  alloy, 
(2)  the  structure  of  hyper-eutectic  alloys  consisting  of  crystalline  particles  of  the 
other  metal  and  of  eutectic  alloy,  and  (3)  the  structure  of  eutectic  alloys  consisting 
of  a  finely  divided  aggregate  of  minute  particles  of  both  metals.  These  three  types 
are  represented  diagrammatic-ally  in  Fig.  401.  Eutectic  alloys  are  often  made  up  of 
very  thin  alternate  and  parallel  plates  or  lamella;  of  each  of  the  two  constituents, 
but  in  some  cases  they  consist  of  rounded  or  elongated  particles  of  one  of  the  con- 
stituents embedded  in  a  matrix  of  the  other  constituents.  The  structures  of  some 


CHAPTER  XXV  —  CONSTITUTION   OF   METALLIC  'ALLOYS 


423 


eutectic  alloys  are  shown  in  Figures  402  to  404.    It  will  be  noted  that  the  constituents 
of  eutectic  alloys  are  not  always  pure  metals. 

Binary  Alloys  whose  Component  Metals  are  Partially  Soluble  in  each  Other  when 
Solid.  —  Metals  are  seldom  absolutely  insoluble  in  each  other  when  solid,  each  metal 


Fig.  403. — :Eutcrtio  (euteotoid)  alloy  of  iron  and  Fe3C.     Pcarlite. 
Magnified  1000  diameters.     (Law.) 


Fig.  404.  —  Kutt'ctir.  alloy  of  SnCu,  and  Cu3P.     Magnified  1000 
diameters.       (Law.) 

in  the  majority  of  cases  being  capable  of  retaining  in  solid  solution  a  small  percentage 
of  the  other  metal.  The  modifications  which  such  partial  solubility  introduces  in  the 
fusibility  curve  should  be  considered.  Let  us  suppose  two  metals  M  and  M'  and  let 
us  assume  that  the  metal  M'  is  capable  of  retaining  in  solution  immediately  after 


424 


CHAPTER   XXV  —  CONSTITUTION   OF   METALLIC   ALLOYS 


solidification,  i.e.  at  the  eutectic  temperature,  5  per  cent  of  the  metal  M  and  that  the 
metal  M  can  retain  in  solid  solution  10  per  cent  of  the  metal  M'.  The  fusibility  curve 
of  these  alloys  constructed  in  the  usual  way  will  have  the  appearance  indicated  in 


I 


O 

1 


4 

M  %      03 
M'%  tOO 


60 

Composition. 

Fig.  405.  —  Typical  fusibility  curve  of  alloys  whose  component  metals  are  partially  soluble  in  each 

other  in  the  solid  state. 


,o 

i  Sol  id  solution 

—  1 
So/id   solution 

*j^ 

A/7     -J-    3  O/     /\  /J                         / 

h/t      ,    £?•   o/      h/l  * 

*s. 

i     '  "^  '    '  o  /o  /  VY             /A 

^.                                     >''    "*u      /O/*'/ 

9 

1                                                                 / 

rNv                                                           c 

«, 

Oi                                      /n 

fills.                                        -O 

5 

o 

•"                         A 

W                              /        Eiyi-ec-, 

f-fc         flflK                                                   "i 

o 

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A  /I                               V                                                                                           X^ 

So. 

SI                                                                                                     '          O         / 

0  M               llv                                  ° 

— 

0 

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I. 

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s.                                     "  **" 

0 

~A  '                   /{* 

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jj 

<o|       /[ 

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i    ytfl 

1  mhv 

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[TTK 

o 

l/l 

1  1  rr>j 

M  °/o   c 

3                ao                   4o 

6O                         QO               94     IOO 

M'°/o  100  97                   SO                            60                           -40                          2.0                   GO 

Fig.  406.  —  Diagram  showing  the  structural  composition  of  binary  alloys  whose  component 
metals  are  partially  soluble  in  each  other  in  the  solid  state. 

Figure  405.  By  comparing  it  with  the  fusibility  curve  of  metals  entirely  insoluble 
(Fig.  396)  it  will  be  noted  that  the  only  differences  between  them  are  (1)  that  the 
eutectic  line  SES'  instead  of  extending  over  the  whole  length  of  the  diagram  now  st  <  >\  >s 
at  the  points  S  and  S'  corresponding  respectively  to  5  per  cent  of  metal  M  and  to  10 


CHAPTER   XXV  —  CONSTITUTION   OF   .METALLIC   ALLOYS 


425 


per  cent  of  metal  M',  and  (2)  the  introduction  of  the  branches  SA  and  S'B  indicates  the 
changes  of  solubility  of  the  metals  as  the  alloys  cool  from  the  eutectic  to  atmospheric 
temperature.  These  curves  show  that  at  atmospheric  temperature  M'  retains  in 
solution  3  per  cent  of  37  and  that  M  retains  in  solution  6  per  cent  of  M'.  LEL'  is 
the  liquidus,  LSES'L'  the  solidus.  Clearly,  any  alloy  containing  less  than  5  per  cent 
of  the  metal  M  or  less  than  10  per  cent  of  the  metal  M'  solidifies  as  a  solid  solution, 
the  diagram  showing  the  absence  of  eutectic  alloys.  In  other  words,  alloys  contain- 
ing from  0  to  5  per  cent  of  M  may  be  considered  as  alloys  of  M'  and  of  a  solid  solution 
of  .17'  plus  5  per  cent  of  .17,  LM'  being  the  liquidus  and  LS  the_sqlidus  of  such  alloys 
and  their  solidification  taking  place  as  explained  in  the  case  of  any  two  metals  form- 
ing solid  solutions.  And,  likewise,  alloys  with  less  than  10  per  cent  of  M'  may  be  re- 
garded as  alloys  of  the  metal  M  and  of  a  solid  solution  of  M  plus  10  per  cent  of  M', 
their  liquidus  being  represented  by  L'M  and  their  solidus  by  US'.  For  all  alloys 
containing  between  5  and  90  per  cent  of  M  the  diagram  shows  that  a  eutectic  alloy  is 
formed  and  that  the  mechanism  of  the  solidification  is  apparently  the  same  as  the  one 

10  SO  3O  +O          SO        €O      JO      QO     SO    IOO  %  Cu 


ooo 
coo. 
700 
coo 

100 


1084-°, 


962 


SolifL  soiuiion.  A.  + 
Solid  solution,  B 


o        10       20      30      •to       so      eo       70       ao     so      too 

A  torn. °/a  Cu . 

Fig.  407.  —  Fusibility  curve  of  alloys  of  silver  and  copper. 

(Desch.) 

described  in  the  case  of  alloys  of  insoluble  metals.  It  should  be  noted,  however,  that 
the  two  components  of  these  alloys  are  no  longer  the  pure  metals  but  two  solid  solu- 
tions, namely,  a  solution  of  M'  containing  5  per  cent  of  M  and  a  solution  of  M  con- 
taining 10  per  cent  of  M',  one  of  these  solid  solutions,  therefore,  crystallizing  when  the 
temperature  of  the  alloy  reaches  any  point  on  the  lines  LEL'  and  the  eutectic  alloy 
being  made  up  of  a  fine  conglomerate  of  these  two  solid  solutions.  To  clarify  let  us 
consider  an  allo^  represented  by  the  point  R,  Figure  405.  At  N  its  solidification  be- 
gins, crystals  of  a  solid  solution  of  M'  containing  5  per  cent  of  M  being  formed.  At  P 
the  residual  molten  mass,  having  reached  the  eutectic  ratio,  crystallizes  at  a  constant 
temperature  as  a  fine  aggregate  of  the  two  solid  solutions.  Since  the  mutual  solubili- 
ties of  the  metals  M  and  M'  decrease,  however,  as  the  alloy  cools  below  the  eutectic 
temperature,  it  is  evident  that  each  crystal  must  undergo  a  gradual  transformation. 
These  transformations  are  indicated  by  the  branches  SA  and  S'B.  After  the 
solidification  of  the  eutectic,  all  alloys  are  aggregates  of  the  two  solid  solutions  whose 
compositions  are  represented  by  the  points  S  and  S',  that  is,  in  the  case  under  con- 
sideration, arbitrarily,  M'  plus  5  per  cent  of  M  and  M  plus  10  per  cent  of  M'.  At  atmos- 
pheric temperature  all  alloys  are  aggregates  of  two  solid  solutions  whose  compositions 
correspond  to  the  points  A  and  B,  that  is  in  the  case  considered  M'  plus  3  per  cent  of  .17 


426 


CHAPTER   XXV  —  CONSTITUTION   OF   METALLIC   ALLOYS 


and  M  plus  6  per  cent  of  .I/'.  At  any  point  below  the  eutectic  line,  the  corresponding 
alloy  is  an  aggregate  of  two  solid  solutions  whose  compositions  are  indicated  by  the 
corresponding  points  on  the  branches  SA  and  S'B.  At  ( '.  for  instance,  in  case  of  alloy  R, 
the  structure  is  composed  of  free  crystals  of  solid  solution  D  and  of  eutectic,  the  com- 

'*&*$*#** 


.f> 


^^^^^^^-' 

Fig.  408.  —  Silver-copper  alloy.  Copper  15  per  cent. 
Magnified  600  diameters.  Dark  constituent  is 
silver  containing  a  little  copper.  (Osmond. j 


Fig.  409.  —  Silver-copper  alloy  eutectic. 
Copper  28  per  cent.  Magnified  600 
diameters.  (Osmond.) 


Fig.  410.  —  Silver-copper  tilloy.  Copper  (>.">  per  cent. 
Magnified  000  diameters.  Light  constituent  is 
copper  containing  a  little  silver.  (Osmond.) 


ponents  of  which  are  solid  solution  D  and  solid  solution  F.  In  other  words,  as  the  alloy 
cools  from  the  eutectic  line  to  atmospheric  temperature,  the  composition  of  its  two 
constituents  shifts  respectively  from  S  to  .1  and  from  8'  to  R. 

The  structural  composition  of  alloys  whose  component  metals  are  partially  soluble 
when  solid  may  be  represented  in  the  usual  way  as  shown  in  Figure  406.   Its  interpre- 


CHAPTER   XXV  —  CONSTITUTION   OF   METALLIC   ALLOYS  427 

tation  does  not  call  for  further  elaboration.  Between  0  and  3  per  cent  of  M  and 
between  0  and  6  per  cent  of  M',  solid  solutions  are  formed  of  corresponding  composi- 
tions. Between  3  and  40  per  cent  of  M,  the  free  solid  solution  formed  is  saturated  with 
.]/.  and  between  40  and  94  per  cent  of  M  it  is  saturated  with  M',  while  the  eutectic 
is  made  up  of  the  two  saturated  solutions.  The  fusibility  curve  of  silver-copper  alloys 
is  shown  in  Figure  407  as  an  example  of  alloys  whose  components  remain  partly  solu- 
ble in  each  other  after  solidification  and  typical  structures  of  these  alloys  are  given 
in  Figures  408  to  410. 


CHAPTER   XXVI 


EQUILIBRIUM  DIAGRAM   OF  IRON-CARBON  ALLOYS 

Fusibility  Curve  of  Iron-Carbon  Alloys.  —  Steel  and  cast  iron  are  essentially  alloys 
of  iron  and  carbon,  and  their  fusibility  curve  or  equilibrium  diagram  may  be  con- 
structed as  in  the  case  of  any  binary  alloy,  namely,  by  determining  the  independent 
cooling  curves  of  a  number  of  alloys  of  the  series  and  plotting  the  evolutions  of  heat 
/Soo  _ 


looo 
Percent  C     O 

Percent  Fe3C  O 


/.O        1.7  2.0  3.0  4.04.3        JTO  6.O        6.67 

A5"  30  45  60  7^  9O         IOO 

Fig.  411.  —  Fusibility  curve  of  iron-carbon  alloys. 


noted  against  the  corresponding  temperatures.    The  resulting  curve  is  shown  in  the 
diagram  of  Figure  411. 

While  it  is  not  generally  possible  for  molten  iron  slightly  above  its  melting-point 
to  dissolve  more  than  5  per  cent  of  carbon,  the  diagram  has  been  constructed  so  as  to 
include  a  percentage  of  carbon  up  to  6.67  per  cent  which  corresponds  to  100  per  cent, 
Fe3C.  That  portion  of  it,  however,  corresponding  to  more  than  5  per  cent  of  carbon, 
has  been  drawn  in  dotted  lines.  The  complete  equilibrium  diagram  should  include 
the  evolutions  of  heat  occurring  after  solidification,  known  as  the  critical  points, 
which  have  been  fully  described  in  these  chapters,  but  they  are  purposely  left  out  of 
the  diagram  of  Fig.  412,  it  being  desired  first  to  confine  our  attention  to  the  mechanism 

428 


CHAPTER    XXVI  — EQUILIBRIUM    DIAGRAM   OF   IROX-CARBOX   ALLOYS     429 


of  the?  solidification  of  iron-carbon  alloys.  In  view  of  the  explanation  of  the  meaning 
of  fusibility  curves  given  in  Chapter  XXV,  it  will  be  evident  that  iron  and  carbon  al- 
loys are  partially  soluble  in  each  other  when  solid,  that  LEU  is  their  liquidus  and 
LSS'  their  solidus  and  that  the  point  E  indicates  the  formation  of  a  eutectic  alloy. 
As  carbon  increases  from  0  to  about  1.70  per  cent,  the  alloys  solidify  as  solid  solutions 
of  corresponding  carbon  content,  LA  being  the  liquidus  and  LS  the  solidus.  These 
solid  solutions  considered  as  microscopical  constituents  are  called  austenite.  The  so- 
lidification of  alloys  containing  between  1.7  and  4.3  per  cent  carbon  begins  when  their 
temperature  reaches  the  line  LE,  crystals  of  a  solid  solution  containing  1.70  per  cent 
carbon  (sometimes  called  saturated  austenite),  being  then  formed  which  keep  on 
growing  until  the  line  SS',  temperature  1130  deg.,  is  reached  when,  clearly,  a  eutectic 


/~/ypo  -e  ui~e  eft  c    a  /  '  /ot/s 

Hyper-  eufecfic 

/oo 

Hypo  . 

f-/yper-  o  u  i~e  c  t'<p  id  alloys 

/ 

\ 

/=>rc 

^-eutecfic             A 

V     P'ro-evfect'i  c 

Safurafed              / 

V    Cement/  fe 

c 

Ausfen  ife          /f[ 

.0     75 

A 

"  -N. 

/ 

5 

/!' 

o 

A\\£ufm^'f*f       ^ 

|. 

Sol  lu   \SOIUi  tOD 

/\\\  \  *-  t//  e 

i  r\ 

O      ,- 

(A    sf&     YeJ 

A^afurafed 

Ausfenite  \ 

o   5o 

A     •+  Cemer 

,f/yej           \" 

~~Q 

t 

K 

£ 

X 

' 

•K 

/ 

\ 

S«5 

i 

V 

2s 

y 

y' 

Ik 

o 

k 

%C      o             '/.o             >.o 

so           4.o 

^O                6.O           6.67 

°/o/~£3C   O                  A5~                3O 

•45               60                 75                90            too 

Fig.  412.  —  Structural  composition  of  iron-carbon  alloys  immediately  after  their  solidification. 

is  formed  composed  of  that  solid  solution  and  of  another  constituent.  The  nature  of 
the  other  constituent  present  in  the  eutectic  alloy  formed  during  the  solidification  of 
iron-carbon  alloys  has  been  in  dispute.  It  seems  at  first  natural  to  infer  that  elemental 
carbon,  i.e.  graphite,  is  that  constituent,  in  which  case  the  eutectic  alloy  would  be 
made  up  of  minute  crystalline  particles  alternately  of  saturated  austenite  and  of 
graphite.  Many  evidences,  however,  point  to  carbon  being  dissolved  in  molten  iron 
as  the  carbide  Fe3C  (cementite)  and  to  its  always  solidifying  as  Fe3C,  although,  as 
later  explained,  it  may  break  up  into  iron  and  graphite  (Fe3C  =  3Fe+C)  immediately 
after  its  solidification.  If  this  hypothesis  be  correct,  it  follows  that  the  eutectic  alloy 
must  be  a  mechanical  mixture  of  minute  particles  of  saturated  austenite  and  of  FeaC. 
It  would  seem  at  first  as  if  the  microscopical  examination  of  alloys  of  suitable  compo- 
sitions should  readily  reveal  the  nature  of  the  eutectic  alloy.  It  will  soon  be  seen, 
however,  that  both  cementite  and  graphite  are  generally  found  in  solidified  eutectifer- 
ous  alloys,  the  microscopical  test  leaving  us  in  doubt  as  to  which  of  the  two  constitu- 


430     CHAPTER   XXVI  — EQUILIBRIUM   DIAGRAM   OF   IROX-CARBON   ALLOYS 

ents  formed  first.  In  the  author's  opinion  it  seems  more  probable  that  when  an  iron- 
carbon  alloy  containing  more  than  some  1.7  per  cent  carbon  solidifies,  a  eutectic  of 
saturated  austenite  and  of  cementite  is  always  produced,  or  in  other  words,  that  in 
the  diagram  of  Figure  411  the  curve  L'  E  indicates  the  crystallization  of  cementite  and 
not  of  graphite.  The  opposite  view  will  be  considered  later.  Let  us  now  follow  the 
solidification  of  four  typical  alloys,  namely,  R,  /(",  K" ',  R'",  containing  respectively 
1  per  cent,  3  per  cent,  4.3  per  cent,  and  4.8  per  cent  of  carbon,  the  first  two  being, 
therefore,  hypo-eutectic  alloys,  the  third  the  eutectic  alloy,  and  the  fourth  a  hyper- 
eutectic  alloy.  As  the  alloy  R  cools,  it  begins  to  solidify  at  Af  through  the  formation 
of  crystals  of  a  solid  solution  the  composition  of  which  is  represented  by  the  point  T 
on  the  solidus.  On  cooling  from  M  to  N  these  crystals  grow  through  successive  addi- 
tions of  crystalline  matter,  the  composition  of  which  varies  from  T  to  N  while  the 
composition  of  the  molten  bath  shifts  from  M  to  U,  the  last  drop  solidifying  having 
the  composition  U.  As  soon  as  the  crystalline  matter  is  deposited,  however,  at  least  if 
time  be  given,  diffusion  takes  place  through  each  crystal  so  that  finally  they  are  chem- 
ically homogeneous  and  of  composition  N ,  the  completely  solidified  metal  being  com- 
posed of  homogeneous  crystalline  grains  of  austenite  containing  one  per  cent  carbon. 

At  any  temperature  V  between  the  solidus  and  the  liquidus  the  crystals  in  equilib- 
rium with  the  molten  metal  must  have  the  composition  Q.  It  may  at  least  be  con- 
ceived that  if  the  cooling  through  and  below  the  solidification  period  be  rapid,  the 
crystalline  grains  of  austenite  will  not  be  chemically  homogeneous,  complete  diffusion 
having  been  prevented; 

In  the  case  of  an  alloy  whose  composition  is  represented  by  the  point  R'  in  the 
diagram,  it  begins  to  solidify  at  M'  (some  1230  degrees  ('.),  when  crystals  of  iron 
containing  1.70  per  cent  of  carbon  (saturated  austenite)  begin  to  form,  the  composition 
of  the  molten  metal  meanwhile  shifting  from  M'  to  E.  At  0',  temperature  1130,  the 
residual  molten  mass  has  reached  the  eutectic  composition  and  now  solidifies  at  a 
constant  temperature,  namely,  the  eutectic  temperature,  the  completely  solid  metal 
being  made  up  of  crystalline  grains  of  saturated  austenite  surrounded  by  a  eutectic  alloy 
composed  of  minute  crystals  of  saturated  austenite  and  minute  crystals  of  cementite. 

The  alloy  K"  has  the  eutectic  composition  (4.3  per  cent  carbon).  It  remains 
liquid  until  its  temperature  falls  to  1130  deg.  C.  when  it  solidifies  at  a  constant 
temperature  after  the  fashion  of  eutectic  alloys. 

The  alloy  R'"  contains  more  carbon  than  the  eutectic  alloy.  On  reaching  its 
solidification  point  M'"  crystals  of  cementite  begin  to  form,  increasing  in  size  as  the 
metal  cools  from  M'"  to  ()'"  while  the  composition  of  the  bath  shifts  from  M'"  to  E. 
At  0'"  the  residual  molten  mass  having  now  the  composition  of  the  eutectic  alloy, 
freezes  at  a  constant  temperature,  the  completely  solidified  alloy  being  made  up  of 
crystals  of  cementite  embedded  in  a  ground  mass  consisting  of  the  eutectic  alloy. 

Structural  Composition  of  Iron-Carbon  Alloys  Immediately  after  Solidification.  - 
The  structural  composition  of  iron-carbon  alloys  immediately  after  their  solidification, 
assuming  that  Fe3C  and  not  graphite  forms,  may  be  represented  in  the  usual  way  by 
the  diagram  of  Figure  412.  The  diagram  clearly  shows  (1)  that  alloys  containing  less 
than  1.70  per  cent  of  carbon  are  made  up  wholly  of  solid  solutions,  (2)  that  alloys 
containing  between  1.70  and  4.3  per  cent  carbon  are  made  up  of  decreasing  propor- 
tions of  saturated  austenite  and  increasing  proportions  of  eutectic,  (3)  that  alloys 
containing  exactly  4.3  per  cent  of  carbon  are  composed  entirely  of  eutectic,  and  (4)  that 
alloys  containing  more  than  4.3  per  cent  carbon  contain  an  increasing  amount  of  free 


CHAPTER   XXVI  —  EQUILIBRIUM   DIAGRAM   OF   IROX-CARBON   ALLOYS     431 


cementito,  and  decreasing  amount  of  euteetic  the  latter  disappearing  altogether  when 
the  metal  contains  6.67  per  cent  carbon.1 

A  modified  form  of  the  structural  composition  diagram  may  profitably  be  combined 

/Soo 


o 

I 

o 


loo 


7-5 


SO 


2-5 


Auste 


D 
O 


C  F>  H 

2  3  <4 

f~'erceni~     Carbon. 


F" 
6.67 


Fig.  413.  —  Fusibility  curve  and  structural  composition  diagram  of  iron-carbon  alloys. 

with  the  equilibrium  diagram  as  shown  in  Figure  413.  Its  interpretation  should  be 
obvious.  The  area  A  BCD  represents  the  structural  composition  of  all  alloys  contain- 
ing less  than  1.7  per  cent  of  carbon,  and  shows  that  they  are  made  up  of  100  per  cent 

1  Howe  questions  the  existence  below  solidification  of  pro-eutectic  cementite  on  the  ground  that  it 
graphit  i/.cs  as  soon  as  formed  unless  the  cooling  be  extremely  rapid  or  considerable  manganese  present. 


432     CHAPTKK    XXVI  —  EQUILIBRIUM    DIAGRAM   OF   IRON-CARBON   ALLOYS 

of  austenite.  By  dividing  this  area  by  the  line  AC,  however,  it  is  further  shown  graphi- 
cally that  the  composition  of  the  austenite  of  these  alloys  varies,  and  that  they  may  he 
considered  as  being  made  up  of  ABC  saturated  austenite  diluted  by  ADC  pure  iron, 
clearly  indicating  that  with  0  carbon  the  alloy  is  entirely  made  up  of  iron  and  with 
1.7  per  cent  carbon  entirely  of  saturated  austenite.  The  area  BCH  represents  the 
proportion  of  free  (pro-eutectic)  saturated  austenite  formed  during  the  solidification 
of  any  alloy  containing  between  1.70  and  4.3  per  cent  carbon.  The  area  BEFH  rep- 
resents the  proportion  of  eutectic  present  in  any  alloy  containing  more  than  1.7  per 
cent  of  carbon.  By  dividing  this  area  in  two  portions,  moreover,  by  means  of  the  line 
BF  we  show  graphically  the  relative  proportions  of  saturated  austenite  and  of  cenien- 
tite  in  the  eutectic.  Finally,  the  area  EOF  indicates  the  percentage  of  pro-eutectic 


Temperature 
iv>  oi  *  < 

0  0  °  < 
0  0  0  c 

\Xx 

^       NX^O                                                                                                                                               / 

^P 

^                                      // 

\                                              V-*                                                                                                                                                                                                    W 

v                   y 

\              *^  ~9"i                      /** 

\                                  o**  ^  ""*^                                        ("O/ 

\                                                        ~~^?      •?       '                                                     / 

Z' 

IIQO 
IOO6 

Au  $  ~/~e  ni  te  -&  ropht  f~&    eui~e  ofic,     sol  i<Jtf  /  e$. 

Per  cent  C  < 
Per  cent^C  ( 

1 

•J                 10             20             JO            4O             SO             60       6 
•)               t5            JO             4S            6O              75            90       1 

"ig.  414.  —  Iron-graphite  fusibility  curve  of  iron-carbon  alloys. 

67 
00 

cementite  in  any  alloy  containing  more  than  4.3  per  cent  carbon.  To  clarify,  let  us 
consider  the  alloy  of  composition  R  (3  per  cent  carbon).  As  it  cools  from  M  to  N  an 
amount  of  saturated  austenite  crystallizes,  represented  in  percentage  by  the  line  OP 
of  the  structural  composition  diagram.  At  N  an  amount  of  eutectic  alloy  is  formed, 
represented  by  the  distance  KO  made  up  of  KL  per  cent  of  cementite  and  LO  per  cent 
of  saturated  austenite. 

The  percentage  of  cementite  and  of  saturated  austenite  in  the  eutectic  may  be 
readily  calculated  by  solving  the  equations 

(1)  A  +  Cm  =  100 

(2)  -      A  +  -    -  Cm.  =  4.30 

in  which  A  represents  the  percentage  of  austenite,  and  Cm  the  percentage  of  cementite 
in  the  eutectic  alloy  and  which  express  the  facts  that  the  carbon  present  in  the  eutectic 


CHAPTER  XXVI  — EQUILIBRIUM   DIAGRAM   OF   IRON-CARBON  ALLOYS     433 

(4.3  per  cent)  is  divided  between  the  austenite  and  the  cementite,  the  former  contain- 
ing 1.7  per  cent  carbon,  the  latter  6.67  per  cent.  The  resolution  of  these  equations 
shows  that  the  eutectic  alloy  contains  nearly  47.7  per  cent  of  saturated  austenite  and 
52.3  per  cent  of  cementite.  The  structural  composition  of  any  iron-carbon  alloy 
ininii'tlidtcly  after  its  solidification  may  likewise  be  readily  calculated.  If  it  contains 
less  than  1.70  per  cent  carbon  it  is  entirely  made  up  of  austenite.  If  more  highly 
carburized,  two  cases  are  to  be  considered:  (1)  the  alloy  contains  between  1.7  and  4.3 
per  cent  carbon  when  it  is  made  up  of  saturated  austenite  (A)  and  of  eutectic  (E)  and 
(2)  the  alloy  contains  between  4.3  per  cent  and  6.67  per  cent  carbon  when  it  is  com- 
posed of  cementite  (Cm)  and  of  saturated  austenite  (A). 

/soo. 


/4OO 


1300  . 


« 

k 

a 

t, 

(3     I3.OO 

I, 

I) 

I 

L^ 
s   noo    \ 


IOOO 

Percent   C  O 

Percent  Fe,C      O 


Austen  i  te-  cem  e  r>f il '  e  eu1~ec,1~i  c.    sol  id  i  fie\5. 


i.o 

IS 


2.O 
30 


J.O 
4-S 


4.Q 
GO 


SO 

75 


GO 
9O 


667 
/OO 


Fig.  415.  —  Combined  graphite-cementite  fusibility  curves  of  iron-carbon  alloys. 

In  the  first  instance  the  two  following  equations 

(1)  A  +  E  =  100 


100 


100 


will  give  the  values  of  A  and  E  for  any  known  carbon  content  (C)  while  in  the  latter 
case  the  equations 

(1)  Cm  +  E  =  100 


100 


100 


will  likewise  give  the  values  of  Cm  and  E. 

An  alloy,  for  instance,  containing  3  per  cent  of  carbon  will  be  found  to  contain  50 
per  cent  of  eutectic  and  50  per  cent  of  saturated  austenite,  while  an  alloy  with  5  per 
cent  carbon  is  composed  of  70.5  per  cent  of  eutectic  and  29.5  per  cent  free  cementite. 


434     CHAPTER   XXVI  — KQI'ILIBHIUM    DIAGRAM   OF   IRON-CARBON   ALLOYS 

Iron-Graphite  Fusibility  Curve.  —  It  has  already  been  mentioned  that  some 
writers  claim  that  graphite  instead  of  ceinentite  may,  and  if  time  be  given  docs,  form 
on  solidification;  in  other  words,  that  the  eutectic  alloy  may  be'  composed  of  saturated 
austenite  and  graphite,  and  that  free  graphite  may  separate  during  the  solidification 
of  alloys  containing  more  than  4.3  per  cent  carbon.  The  diagram  interpreting  this 
assumption  which  may  be  called  the  iron-graphite  fusibility  curve  is  shown  in  Figure 
414.  It  will  be  seen  to  be  similar  to  the  iron-cementite  diagram  (Fig.  411). 

Combined  Graphite-Cementite  Diagram.  —  Recognizing  the  possibility  of  the 
formation  of  a  graphite-austenite  eutectic  and  of  a  cementite-austemte  eutectic  ac- 
cording to  the  rate  of  cooling,  some  writers,  notably  ('harpy  and  Crenel,  Heyn,  and 
Benedicks,  recommend  the  use  of  double  solidification  curves  in  the  equilibrium  dia- 
gram of  iron-carbon  alloys.  These  double  curves  are  shown  in  Figure  415,  the  dotted 
lines  referring  to  the  austenite-graphite  system.  It  will  be  noted  that  free  graphite 
and  the  graphite-austenite  eutectic  form  respectively  at  temperatures  slightly  higher 
than  those  at  which  free  ceinentite  and  the  cementite-austenite  eutectic  form,  the 
solidification  of  the  latter  constituents  being  regarded  as  due  to  surfusion  or  under- 
cooling. It  is  accordingly  believed  that  only  the  iron-graphite  system  is  stable,  the 
iron-cementite  system  being  "metastable."  Our  reasons  for  believing  that  graphite 
and  not  cementite  is  the  final  condition  to  be  assumed  by  carbon  are  based  on  repeated 
and  concordant  observations  that  any  condition  promoting  stable  equilibrium  results 
in  the  transportation  of  cementite  into  graphite  as,  for  instance,  very  slow  cooling 
during  and  below  solidification  or  long  exposure  of  cementite  (as  in  the  manufacture 
of  malleable  cast-iron  castings)  to  a  high  temperature,  while  on  the  contrary,  treat- 
ments opposing  equilibrium,  such  as  quick  cooling,  always  result  in  the  formation  or 
retention  of  cementite.  Roozeboom,  when  he  first  took  up  the  study  of  the  iron- 
carbon  diagram,  believed  that  cementite  was  the  final  stable  condition  of  carbon,  any 
graphite  having  formed  during  solidification  combining  with  iron  at  some  1000  degrees 
C.  to  form  cementite.  The  error  of  this  view  soon  became  apparent,  however,  to 
Roozeboom  himself. 

Graphitizing  of  Cementite.  —  Although  recognizing  the  fact  that  graphite  and  not 
cementite  must  be  the  final  condition  assumed  by  carbon,  the  author  believes  with 
some  other  observers  that  graphite  never  forms  directly  as  iron-carbon  alloys  solidify, 
its  occurrence  always  resulting  from  the  breaking  up  of  cementite  according  to  the 
reaction 

Fe3C  =  3Fe  +  C 

from  which  it  would  follow  that  the  iron-graphite  fusibility  curve  need  not  be  in- 
cluded in  the  equilibrium  diagram.  Even  those  who  believe  in  the  possibility  of  the 
direct  formation  of  graphite  do  not  deny  that  ceinentite  is  the  constituent  which 
generally  forms  first  on  solidification;  they  state  that  the  separation  of  graphite  from 
molten  iron  is  possible  only  in  the  case  of  very  slow  cooling.  As  a  matter  of  fact,  how- 
ever, they  offer  no  conclusive  evidences  that  such  separation  ever  takes  place.  From 
the  formation  of  "kish,"  that  is,  of  graphite  floating  on  the  surface  of  a  ladleful  of 
molten  cast  iron,  it  does  not  follow  that  such  graphite  formation  was  not  preceded  by 
the  formation  of  cementite.  If,  on  very  slow  cooling,  graphite  separated  directly  from 
molten  iron,  the  bulk  of  it  at  least  should  rise  to  the  top  of  the  molten  bath  and  the 
solidified  mass  should  be  found  much  richer  in  graphite  near  its  surface  than  at  some 
distance  from  it.  The  author  does  not  understand  such  heterogeneity  in  the  dis- 
tribution of  graphitic  carbon  to  be  observed  in  the  case  of  gray  cast-iron  cast- 


CHAPTKH  XXVI  — EQUILIBRIUM   DIAGRAM   OF  1RON-CARBOX  ALLOYS     435 

ings.1  On  the  contrary,  on  the  assumption  that  graphite  results  from  the  breaking  up  of 
cementite  soon  after  its  solidification,  it  is  readily  understood  why,  in  spite  of  their  very 
great  difference  in  specific  gravity,  iron  and  graphite  are  found  uniformly  distributed  in. 
the  various  parts  of  castings.  The  microscopical  examination  of  the  structure  of  very 
.•slowly  cooled  castings  does  not  reveal  the  existence  of  a  graphite-austenite  eutectic. 

That  cementite  is  unstable,  being  readily  converted  into  iron  and  graphitic  carbon, 
is  also  generally  admitted.  It  is  upon  this  instability  of  cementite  that  the  important 
industrial  operation  of  converting  white  cast-iron  castings  into  graphitic  malleable 
castings  is  based.  And  there  is  abundant  evidence  that  the  higher_the  temperature, 
the  more  readily  is  cementite  dissociated,  from  which  it  follows  that  the  higher  the 
temperature  at  which  cementite  forms  the  more  readily  will  it  be  converted  into 
graphitic  carbon.  Bearing  this  in  mind,  and  with  the  assistance  of  the  diagram  of 
Figure  3,  let  us  look  more  closely  into  the  graphitizing  of  cementite.  The  diagram 
shows  clearly  that,  during  the  solidification  of  alloys  containing  more  than  1.70  per 
cent  of  carbon,  (1)  some  cementite  forms  as  pro-eutectic  cementite  if  the  metal  con- 
tains more  than  4.3  per  cent  carbon,  (2)  some  cementite  forms  as  eutectic  cementite  in 
all  alloys,  (3)  some  cementite  remains  dissolved  in  the  eutectic-austenite  of  all  alloys, 
and  (4)  some  cementite  remains  dissolved  in  the  free  austenite  of  alloys  containing  less 
than  4.3  per  cent  carbon.  Considering  first  the  free  cementite,  that  is,  the  pro-eutectic 
and  the  eutectic  cementite,  it  is  evident  that  the  former  is  formed  at  a  higher  tempera- 
ture, and  that  the  more  carbon  in  the  alloy  the  higher  the  temperature  at  which  it 
begins  to  form.  It  seems  safe  to  infer,  therefore,  that  pro-eutectic  cementite  will  break 
up  into  graphite  and  ferrite  more  readily  than  eutectic  cementite,  this  being  consistent 
with  the  well-known  fact  that  hyper-eutectic  alloys  are  generally  rich  in  graphite  even 
after  relatively  quick  cooling.  The  presence  of  pro-eutectic  cementite  may  also  pro- 
mote the  formation  of  graphitic  carbon  because  once  this  graphitizing  process  is 
started,  it  is  likely  to  extend,  if  time  be  given,  to  the  bulk  of  the  cementite,  first  the  eu- 
tectic cementite  and  later  the  austenite-cementite  undergoing  the  change.  Alloys  con- 
taining less  than  4.3  per  cent  carbon  and  consequently  free  from  pro-eutectic  cementite 
should  not  become  graphitic  as  readily  because  of  the  lower  temperature  at  which 
eutectic-cementite  forms.  If  a  large  proportion  of  cementite  be  formed,  however, 
that  is,  if  the  alloys  contain  more  than  3  or  3.5  per  cent  of  carbon,  a  certain  amount 
of  graphitizing  is  readily  induced  through  slow  cooling.  With  decreasing  carbon 
the  breaking  up  of  cementite  becomes  progressively  more  difficult  until,  in  alloys  con- 
taining less  than  1.7  per  cent  carbon  (the  steel  series),  and,  therefore  free  from  eutectic 
cementite,  graphitic  carbon  is  very  seldom  formed. 

It  should  be  borne  in  mind  that  while  those  alloys  which  contain  but  a  small  pro- 
portion of  carbon  cannot  be  made  graphitic,  when  a  large  proportion  of  carbon  is 
present,  the  graphitizing  once  started  may  be  made  to  include  the  totality  of  the 
cementite,  thus  explaining  the  freedom  of  steel  from  graphite  and  the  freedom  of 
some  cast  irons  from  cementite. 

The  foregoing  remarks  apply  to  pure  iron-carbon  alloys,  the  influence  of  the 
elements  generally  present  in  commercial  products  having  been  purposely  ignored. 
When  dealing  with  commercial  steel  and  cast  iron,  the  well-known  influence  of  silicon 
in  promoting  the  formation  of  graphitic  carbon  should  be  remembered  as  well  as  the 
opposite1  influence  of  sulphur  and  manganese1.  Because  of  the  presence  of  a  notable 

1  Howe  reports  the  segregation  of  graphite  in  the  upper  part  of  lash-bearing  east  iron,  but  this 
is  readily  explained  on  the  ground  that  such  irons  must  be  liyper-eutectic  in  composition  and  that 
the  pro-eutectic  cementite  beginning  to  graphitize  while  the  iron  is  still  partially  liquid  shows  a 
tendency  To  rise  to  the  top,  hence  indeed  the  formation  of  kish. 


Fig.  417.  —  Magnified  7.~>l>  diameters. 


Fig.  41(1.  —  Magnified  •">()  dimnetora. 


\^.   11'.).  —  Magnified  760  diameteis. 


Fig.  41S.  —  Magnified  ">"  diameters. 


FiK.   IL>CI.     -  .M:igmlu:d."iOdianii-i.T.s.'  Fig.  421.  —  Magnified  760  diametew. 

Figs  41(i  and  417. —  Iron-carbon  alloy.  Hypo-eutcctic.  Structure  immediately  after  solidification.  Dark  crystallite  of 
nturated  aiutenitc  in  a  matrix  of  aurtenlte-oemwitiU  ruti-ctic.  Figs.  41S  and  41!).  -  Iron-carbon  alloy.  Aiisir,,i:,.- 
ccnii'iititi.  rutoctic.  Figs.  420  and  421.  —  Ir.m-<-:trl>.m  alloy.  Hyper-cuteotic.  Strui'ture  iinnifdiati'ly  after  snlidifi- 
cation.  Nil-dies  of  fcinc'iititf  in  a  matrix  of  ailsti'iiiti-cc'inc'iititc  cutoctk-.  (QorODB.) 

436 


CHAPTER    XXVI  —  EQI'ILIBKH  M    DIAGRAM   OF   IKOX-CARbOX   ALLOYS     437 


proportion  of  silicon,  commercial  cast  irons  after  slow  cooling  are  necessarily  more 
graphitic  than  pure  alloys  of  same  carbon  content. 

Structure  of  Iron-Carbon  Alloys  Immediately  after  Solidification.  —  If  the  alloy 
contains  less  than  some  1.70  per  cent  carbon  it  is  made  up  after  complete  solidification 
of  crystalline  grains  of  austenite.  It  has  been  explained,  however,  that  in  the  absence 
of  manganese  or  other  ''retarding"  elements  it  is  not  possible  to  prevent,  even  through 
very  rapid  cooling,  the  transformation  of  some  of  the  austenite  at  least  into  martensite. 
The  polyhedric  structure  of  austenite  has  been  illustrated  in  these  lessons  in  the  case 
of  special  steels  (manganese  and  nickel  steels)  and  it  is  now  welLunderstood  that  the 


r 


6.67 


Fig.  4122.  —  Equilibrium  diagram  of  iron-carbon  alloys. 


frequent  network  structures  of  slowly  cooled  steel  are  due  to  the  existence  of  poly- 
hedral austenitic  structures  above  their  critical  range. 

If  the  alloy  contains  from  1.70  to  4.3  per  cent  carbon  it  is  made  up,  after  solidifi- 
cation, of  crystals  of  saturated  austenite  and  of  eutectic  alloy.  This  is  well  shown 
after  (kurens  in  Figure  416,  in  which  the  dark  "pine  tree"  crystals  consist  of  saturated 
.Mistenite,  while  the  ground  mass  is  the  cementite-austenite  eutectic.1  In  Figure  417 
the  same  structure  is  shown  more  highly  magnified.  If  the  alloy  contains  exactly  4  3 
per  cent  carbon,  it  consists  wholly  of  eutectic  as  shown  under  different  magnifications 
m  Figures  418  and  419.  It  has  been  seen  that,  theoretically,  this  eutectic  contains 
47.7  per  cent,  of  saturated  austenite  (the  dark  constituent),  and  52.3  per  cent  of  cem- 
entite.  Alloys  containing  more  than  4.3  per  cent  carbon  consist  after  solidification 
of  free  cementite  in  the  form  of  needles  embedded  in  a  eutectic  matrix  as  shown  in 
Figures  420  and  421. 

It  should  be  noted  that  the  dark  constituent  occurring  in  these  structures  and 
described  as  saturated  austenite  may  not  be  absolutely  unaltered  austenite  because  of 

1  Wiist  has  suggested  the  name  of  "  Ledcburite"  for  this  eutectic  in  honor  of  the  late  well-known 
German  metallurgist  A.  Ledebur. 


438     CHAPTER   XXVI  —  KQl'ILIBRIUM   DIAGRAM   OF    IROX-CARIiON   ALLOYS 


CHAPTER  XXVI  — I'XjriLIBKH'M    DIACKA.M    OF   JHOX-CARBOX   ALLOYS     439 

the  difficulty  of  completely  preventing  the  transformation  of  that  constituent  even 
in  the  presence  of  a  large  amount  of  carbon  and  by  very  sudden  cooling.  If  the  aus- 
tenite  has  undergone  some  transformation,  however,  so  that  it  contains  some  mar- 
tensite  and  even  troostite  those  transformations  must  have  taken  place  in  situ  and  the 
structures  reproduced  in  Figures  6  to  11  must  represent  accurately  the  structural 
aspect  of  the  corresponding  alloys  after  solidification. 

Complete  Equilibrium  Diagram.  —  In  the  foregoing  pages  only  the  solidification 
curves  of  iron-carbon  alloys  have  been  considered  and  the  probable  mechanism  of 
their  freezing  explained.  Their  equilibrium  diagram,  however,  jnust  include  all  heat 
evolutions  observed  on  cooling  from  the  liquid  condition  to  atmospheric  temperature; 
in  other  words,  the  thermal  critic  ;1  points  fully  described  in  previous  chapters  arc 
part  of  the  complete  equilibrium  diagram  as  indicated  in  Figure  422. 

Since  the  meaning  of  every  curve  of  this  diagram  has  been  discussed,  it  only  re- 
mains to  inquire  into  any  possible  structural  or  other  changes  taking  place  after 
solidification  and  before  the  alloys  reach  their  respective  thermal  critical  point  or 
points,  that  is,  while  they  cool  from  the  solidus  LSS'  to  the  eutectoid  line  CDF.  The 
changes  which  do  or  may  take  place  as  the  alloys  cool  in  this  range  are  clearly  stated 
in  Figure  42:>.  In  this  diagram  the  most  likely  significance  of  every  curve  is  indicated 
as  well  as  the  nature  of  all  structural  transformations,  and  of  all  possible  resulting 
structures  after  complete  cooling.  The  author  believes  that  it  embodies  those  infer- 
ences best  supported  by  analogy  and  by  experimental  evidences.  Although  neces- 
sarily involving  some  repetition,  a  methodical  examination  of  the  various  parts  of 
this  complete  diagram  seems  advisable,  as  it  will  permit  a  recapitulation  of  the  various 
matters  previously  discussed. 

Let  us  consider  (1)  the  solidification  of  iron-carbon  alloys,  (2)  their  cooling  from 
the  solidus  LtfES'  to  the  eutectoid  temperature  CDF,  and  (3)  their  cooling  through 
the  eutectoid  temperature  and  their  final  structures. 

According  to  the  mechanism  of  their  solidification  iron-carbon  alloys  are  divided 
into  three  classes,  namely,  (1)  alloys  containing  less  than  1.70  per  cent  of  carbon,  (2) 
alloys  containing  between  1.70  and  4.3  per  cent  carbon,  and  (3)  alloys  containing 
more  than  4.3  per  cent  of  carbon.  Alloys  containing  less  than  1.70  per  cent  of  carbon 
and  including,  therefore,  all  the  steels  of  commerce  solidify  as  solid  solutions  of  the 
carbide  Fe.(( '  (cementite)  in  gamma  iron,  these  solutions  being  known  as  austenite. 
LA  is  the  liquidus  and  LS  the  solidus  of  these  alloys.  Alloys  containing  between  1.70 
and  4.3  per  cent  of  carbon  solidify  through  the  formation  of  crystals  of  saturated 
austenite  at  gradually  decreasing  temperatures  and  through  the  final  solidification, 
at  the  euteetic  temperature,  of  the  residual  molten  metal  necessarily  of  eutectic  com- 
position. Alloys  containing  more  than  4.3  per  cent  of  carbon  solidify  through  the 
formation  of  cementite  crystals  at  gradually  decreasing  temperatures,  and  through  to 
final  solidification,  at  the  eutectic  temperature,  of  the  residual  molten  metal  necessarily 
of  eutectic  composition. 

In  cooling  below  their  solidus,  LS,  alloys  with  less  than  1.70  per  cent  carbon  undergo 
no  change  until  they  reach  their  thermal  critical  points  Ar3,  Ar3.2,  Ar3.2.i  or  Arcm  as 
the  case  may  be,  when,  if  they  contain  less  than  some  0.85  per  cent  of  carbon  (hypo- 
eutectoid  steels),  some  iron  is  set  free  and  converted  into  beta  iron,  while  if  they 
contain  more  carbon  (hyper-eutectoid  steels),  cementite  is  liberated.  In  either  case 
when  the  eutectoid  temperature  is  reached  the  residual  austenite,  now  of  eutectoid 
composition  (0.85  per  cent  carbon),  is  converted  into  pearlite. 


440     CHAPTER   XXVI  —  EQUILIBRIUM    DIACHAM    OF   1ROX-CARBOX   ALLOYS 

ISOO, 2 ^ 6_ 


F'ro  -  e  u  Te  c  ti  c 
te  \     Cerri entile 


•Structural 
corn-position 
i rrtrn  ec//a/e  /y 

ofter- 
Solid  if  i  cat  I  on 


SfrucTurol 
com  po-s  it/On 
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Structural 
compos  it  ion 
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2  4  6  6.57 

C-arbon  °/o 
Fig.  424.  —  Equilibrium  and  struct unil  composition  cli:is>':uii  of  iron-carbon  allovs. 


CHAPTER  XXVI  —  EQUILIBRIUM   DIAGRAM   OF   IRON-CARBON   ALLOYS     441 

After  solidification,  alloys  containing  between  1.70  and  4.3  per  cent  carbon  are 
aggregates  of  saturated  austenite  (austenite  containing  1.70  per  cent  C  or  25.5  per 
cent  cement  it  i1)  and  of  cenientite-austenite  eutectic.  On  cooling  below  their  solidus, 
cementite  (pro-eutectoid  cementite)  is  liberated  both  from  the  free  and  from  the 
eutectic-austenite,  while  if  the  cooling  be  sufficiently  slow  both  the  eutectic  and  pro- 
eutectoid  cementite  may  be  partly  or  wholly  dissociated  into  graphite  and  iron  (ferrite). 
Indeed,  the  graphitizing  may  even  include  the  eutectoid  cementite,  in  which  case  the 
alloy  is  made  up  solely  of  ferrite  and  graphite. 

Alloys  containing  more  than  4.3  per  cent  of  carbon  are,  immediately  after  solidi- 
fication, aggregates  of  cementite  (pro-eutectic  cementite)  and  of  cementite-austenite 
eutectic.  On  slow  cooling  below  their  solidus,  cementite  (pro-eutectoid  cementite)  is 
liberated  from  the  eutectic  austenite  while  the  pro-eutectic,  eutectic,  and  pro-eutectoid 


Fig.  425.  —  The  author's  early  equilibrium  diagram  (1896). 

cementite  may  be  partly  or  wholly  dissociated  into  graphite  and  ferrite,  in  some  ex- 
treme cases  the  eutectoid  cementite  even  being  graphitized. 

On  cooling  through  the  eutectoid  temperature,  any  remaining  austenite  is  bodily 
converted  into  pearlite. 

The  above  consideration  clearly  shows  that  in  alloys  containing  more  than  some 
1.70  per  cent  carbon  four  types  of  structure  may  be  formed  according  to  the  rate  of 
cooling  below  the  solidus: 

(I)  Cementite  plus  pearlite,  the  structure  of  white  cast  iron,  readily  produced  by 
quick  cooling  and  representing  a  metastable  equilibrium.  (II)  Ferrite  and  graphite, 
an  extreme  case,  possible  only  after  very  slow  cooling  and  in  the  presence  of  much 
silicon,  little  manganese,  and  sulphur,  and  representing  stable  equilibrium.  (Ill)  Cem- 
ent ite  plus  pearlite  and  graphite,  the  structure  of  gray  cast  iron  with  hyper-eutectoid 
matrix,  resulting  from  slow  cooling,  promoted  by  the  presence  of  silicon  and  opposed 
by  sulphur  and  manganese.  (IV)  Pearlite,  ferrite,  and  graphite,  the  structure  of  gray 
cast  iron  with  hypo-eutectoid  matrix,  produced  because  of  slower  cooling  or  because 
of  the  presence  of  more  silicon  or  of  less  sulphur  and  manganese. 


442     CHAPTKH    XXVI  —  EQUILIBRIUM    DIAGRAM   OF   1RON-CAHHOX   ALLOYS 

It  will  be  explained  in  the  next  chapter  that,  according  to  the  phase  rule,  only  two 
components  may  be  present  in  a  binary  alloy  in  a  state  of  equilibrium  from  which 
it  follows  that  gray  cast  irons,  since  they  contain  besides  iron  both  cementite  and 
graphite,  are  out  of  equilibrium.  One  of  the  components  must  disappear  if  the  alloy 
is  to  assume  equilibrium.  Cementite  is  undoubtedly  that  component  as  proven  by 
the  malleablizing  of  cast  iron  when  cementite  is  readily  dissociated  into  graphite  and 
ferrite  on  prolonged  heating  to  a  high  temperature. 

In  Figure  424  the  complete  equilibrium  diagram  is  shown  combined  with  three 
constitutional  diagrams  showing  graphically  the  structural  composition  of  iroii-car- 
bon  alloys  (1)  immediately  after  their  solidification,  (2)  immediately  before  the  eutec- 
toid  temperature,  and  (3)  below  the  eutectoid  temperature,  on  the  assumption  that 
no  graphitic  carbon  is  formed.  The  structural  changes  taking  place  while  the  alloys 


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Fig.  426.  —  Roberts-Austen's  first  equilibrium  diagram  (1897). 

cool  below  their  solidus  down  to  the  eutectoid  temperature  are,  in  this  way,  clearly 
depicted.  The  following  facts,  for  instance,  are  graphically  shown,  (1)  the  pro-eutectic 
cementite  formed  during  the  solidification  of  hyper-eutectic  alloys  and  the  eutectic 
cementite  present  in  all  alloys  containing  more  than  1.70  per  cent  carbon  remain  un- 
changed as  the  alloy  cools  to  atmospheric  temperature,  (2)  the  free  saturated  austenite 
of  hypo-eutectic  alloys,  as  well  as  the  eutectic-austenite,  are  converted  into  eutectoid 
austenite  through  the  liberation  of  cementite  (pro-eutectoid  cementite),  the  area 
EDH  representing  the  cementite  thus  set  free,  and  (3)  in  hypo-eutectoid  alloys  iron 
is  set  free  as  shown  by  the  area  FJG.  The  lower  diagram  shows  that,  in  cooling 
through  the  eutectoid  temperature,  the  remaining  austenite,  now  necessarily  of 
eutectoid  composition,  and  sometimes  called  hardenite  is  converted  into  pearlite. 
Taking,  for  instance,  the  metal  whose  composition  and  temperature  are  represented 
by  the  point  R,  its  transformations  and  final  structure  are  clearly  shown.  At  M  it 
begins  to  solidify  through  the  formation  of  crystals  of  saturated  austenite;  from  M  to 
N  the  austenite  crystals  continue  to  grow,  the  percentage  of  free  austenite  present  in 
the  solidified  metal  being  represented  by  the  distance  OP;  at  N  the  residual  bath 


CHAPTER    XXVI  — EQUILIBRIUM   DIAGRAM    OF   IROX-CARBOX   ALLOYS     443 

solidifies  as  a  eutectic  alloy,  the  percentage  of  which  is  proportional  to  the  distance 
KO;  this  eutectic  contains  KL  =  PQ  =  TU  per  cent  of  cementito  and  LO  per  cent 
of  saturated  austenite;  LI'  is  the  total  amount  of  austenite  in  the  alloy;  after  solidifi- 
cation in  cooling  from  N  to  K,  pro-eutectoid  cementite  is  liberated  both  from  the 
free  and  from  the  eutectic-austenite,  Q8  representing  the  percentage  of  cemeiitite 
finally  expelled;  on  reaching  the  point  K,  the  remaining  austenite,  ST,  is  of  eutectoid 
composition,  when  it  is  sometimes  called  hardenite,  and  in  cooling  through  K  this 
austenite  is  converted  into  pearlite,  the  metal  being  finally  made  up  of  TU  per  cent 
of  eutectic  cementite,  UV  per  cent  of  pro-eutectoid  cementite,  and  VX  per  cent  of 
pearlite,  the  latter  containing  VW  per  cent  of  cementite,  and  WX  per  cent  of  ferrite, 
or  of  TV  per  cent  of  free  cementite  and  VX  per  cent  of  pearlite,  or  again  of  TW 
per  cent  of  total  cementite  and  WX  per  cent  of  ferrite. 

Historical.  —  In  view  of  the  scientific  and  practical  importance  of  the  equilibrium 


3-0     &Z 
CARBON    PER    CENT 


*»    *Z     tti.fi        s        ft    S * 


l-'ig.  427.  —  Roberts-Austen's  second  equilibrium  diagram  (1899). 

diagram  of  iron-carbon  alloys,  a  brief  historical  sketch  of  its  evolution  should  be  of 
interest  to  the  reader.  The  first  diagram  was  published  by  the  author  in  1896.1  It  is 
reproduced  in  Figure  425.  It  will  be  noted  that,  although  the  diagram  includes  only 
the  thermal  critical  points,  it  is  otherwise  substantially  accurate.  In  describing  it 
the  author  wrote  in  part: 

"Figure  1  shows  graphically  the  position  of  the  critical  points  in  cooling  steels  of 
various  carbons.  The  width  of  the  black  lines  does  not  refer  to  the  intensity  of  the 
retardations,  but  only  indicates  the  range  of  temperature  which  they  cover.  For 
instance,  it  shows  that  the  single  retardation  of  high  carbon  steel  begins  at  about 
680  deg.  C.  and  ends  at  about  640  deg.  C.  The  maximum  evolution  of  heat  lies  some- 
where between  these  limits,  but  not  necessarily  in  the  middle. 

'  This  graphical  representation  was  obtained  by  plotting  the  results  of  the  investi- 
gations of  Osmond,  Howe,  Roberts-Austen,  Arnold,  and  the  writer;  and,  with  one  or 
two  exceptions  all  their  figures  fall  very  nearly  within  the  limits  here  indicated." 

1  "The  Microstructure  of  Steel  and  the  Current  Theories  of  Hardening,"  ALBERT  SAUVEDK, 
Transactions  American  Institute  of  Mining  Engineers.  1896,  p.  867. 


444     CHAPTER  XXVI  —  EQUILIBRIUM    DIAGRAM    OF   IRON-CAKBOX    ALLOYS 


It  is  from  this  modest  beginning  that  the  present  diagram  was  evolved.1 
In  1897  Roberts-Austen  published  in  his  fourth  report  of  the  Alloys  Research  Com- 
mittee of  the  Institution  of  Mechanical  Engineers  the  diagram  reproduced  in  Fig.  426. 
Two  years  later,  in  1899,  the  diagram  shown  in  Fig.  427  was  published  by  Roberts- 
Austen  and  Stansfield  in  the  fifth  report  of  the  Alloys  Research  Committee.  Some  of 
the  conspicuous  features  of  this  diagram  should  be  noted.  The  solidification  point  of 
pure  iron  was  indicated  to  be  1600  degrees  C.  whereas  we  know  now  that  it  is  nearly 
1500  deg.  No  attempt  had  been  made  yet  at  ascertaining  the  end  of  the  solidification, 
that  is,  the  solidus,  of  alloys  forming  solid  solutions;  the  formation  of  a  eutectic  on 
solidification  was  indicated  as  taking  place  in  alloys  containing  more  than  one  per  cent 
carbon;  graphite  was  supposed  to  crystallize  during  the  solidification  of  alloys  con- 


'•too' 


soo 


000* 


Warte  isite 


Fsrr'le* 


PerUff 

3L- 


-P* 


*** 


Ma 


Ci  'rbon  Per  3 


Wart,  visit* 


~!ra/t  hike 


•nfrJ-s 


T.is 


=r—2 


Liquid. 
yCrafi/iite 


F' 


a 
& 


R 


Fig.  428.  —  Roozeboom's  equilibrium  diagram  (1900). 

taming  more  than  4.3  per  cent  carbon,  there  being  in  the  diagram  no  indications  of  pos- 
sible formation  of  cementite;  the  eutectic  alloy  was  assumed  to  be  a  graphite-iron  eu- 
tectic ;  critical  points  occurring  below  the  eutectoid  temperature  were  represented  in  the 
diagram  and  marked  "hydrogen  points"  (see  Chapter  X,  "Minor  Critical  Points"); 
the  Arcm  curve  was  arbitrarily  extended  to  yield  a  V-shaped  curve.  Roberts- Austen 
mentioned  the  formation  of  a  solid  solution,  free  in  hypo-eutectic  steels,  and  as  a  con- 
stituent of  the  eutectic  in  alloys  of  eutectic  composition,  and  he  ascribed  the  presence 
of  free  cementite  in  cast  iron  to  the  liberation  of  that  constituent  from  solid  solution. 
In  1900  Roozeboom  took  up  the  study  of  Roberts-Austen's  diagram,  and  applying 
to  it  the  teachings  of  the  phase  rule  published  the  diagram  of  Fig.  428  as  a  probably 
accurate  representation  of  the  solidification  mechanism  of  iron-carbon  alloys  and  of 
the  structural  transformations  taking  place  after  solidification.  In  this  diagram,  the 
line  6a,  that  is,  the  solidus  of  alloys  forming  solid  solutions,  is  for  the  first  time  indicated ; 

1  Since  these  remarks  were  written  Prof.  H.  M.  Howe  has  expressed  the  opinion  that  credit  for 
the  first  equilibrium  diagram  is  due  to  Rcinhard  Mannosmann. 


CHAPTER   XXVI  — EQUILIBRIUM    DIAGRAM   OF   IROX-CARBOX    ALLOYS     445 


1500 


X? 


5 


X 


I40O 


X 


S 


2 


1300 


X 


1200 


1 


kl 
0 


5  "00 

f- 
z 


«)10' 
li 

ft 

D 

< 

Jj     900 

I 

kl 
H 


^ 


800 


CARBON       PER     CENT 

Tig.  429.  —  Carpenter's  and  Kcrlinij's  equilibrium  diagram  (190-1). 


446    CHAPTER  XXVI  —  EQUILIHUH  M    D1.U1KAM   OF   IRON-CARBON   ALLOYS 


the  word  martensite  is  used  instead  of  austenite  to  denote  the  solid  solution  of  iron 
and  carbon;  free  graphite  is  assumed  to  form  during  the  solidification  of  alloys  con- 
taining more  than  4.3  per  cent  carbon  and  the  constituents  of  the  eutectic  alloy  are 
supposed  to  be  martensite  (solid  solution)  and  graphite.  The  Ar(.m  curve  of  hyper- 
eutectoid  steels  is  arbitrarily  extended  as  an  horizontal  line  starting  from  E  (1.75  per 
cent  carbon  and  1000  deg.  C.)  and  extending  to  the  end  of  the  diagram.  Roox,eboom 
argued  that,  while  graphite  formed  on  solidification  in  all  alloys  containing  more  than 
2  per  cent  carbon,  this  graphite  at  1000  deg.  (line  EF)  recombined  with  iron  to  yield 
cementite  so  that,  finally,  alloys  in  equilibrium  would  contain  only  ferrite  and  cementite, 
thus  conforming  to  the  phase  rule  which  forbids  the  presence  of  more  than  two  com- 
ponents in  binary  alloys.  It  has  since  been  conclusively  shown  that  Roozeboom  was 
in  error,  that  while  the  ferrite-cementite  system  is  in  equilibrium  according  to  the 


TEMPER 

ATURE 
1500' 
1400 
1300 

1100 
IOOO 
900 
600 
700 
600 

500 
IF 

A 

^ 

-^s 

-^ 

Li 

au 

D 

\ 

SLI 

^\ 

3UIC 

^^ 

^^^ 

A 

) 
4—±\ 

QtHO 
ENTI'£ 
M  *; 

&   M 
\      < 

XEC 
RSI 

ALS 

"^ 

Ss 

9*' 

0 

r*r\ 

CEM 
YtT 

M 

IXE 

•> 

gr 

__. 

-  —  _ 

__  -  — 

^ 

s  . 

"'%' 

.... 

r- 

'E" 

1 

r> 

f11 

CR\ 

'STA 

•V 

'/ 

1 
1 
1 

U 

'  / 

'     h 

IXE 

)  CR 

YSTfi 

LS 

E' 

JTE 

:TIC 

& 

\ 

1 

&    E 

r"TE 

:TIC 

C 

;MEr 

TITE 

CRY 

STAL^ 

-^ 

'E 

t 

^t 

PER 

»ITE 

p 

EAR 

LITE 

&  c 

EME 

NTI 

re 

PEAR 

LITE 

Pt 

/?  CA 

wr 

?Aff6 

ON 

ION 

Fe 

r             f             j             -f             5             6     CEM 

-TIT 

Fig.  430.  —  Benedicks'  double  equilibrium  diagram. 

phase  rule,  it  is  in  metastable  equilibrium,  the  ferrite-graphite  system  being  the  only 
stable  one.  The  hypothetical  horizontal  line  EF  is  now  consequently  omitted  from 
the  equilibrium  diagram,  and  the  Ar(.m  curve  made  to  join  the  eutectic  line  at  its 
origin  (a). 

In  1904  Carpenter  and  Keeling  made  a  series  of  very  careful  experiments  in  order 
to  ascertain  the  evolutions  of  heat  taking  place  in  cooling  pure  iron-carbon  alloys  from 
the  liquid  state  to  atmospheric  temperature.  By  plotting  their  results,  the  equilibrium 
diagram  reproduced  in  Figure  429  was  obtained.  The  solidification  of  pure  iron  is 
shown  to  take  place  at  1500  deg.  C.  The  curves  arc  otherwise  identical  to  those  of 
Roozeboom,  the  horizontal  line  EF  having  been  introduced.  The  faint  evolutions  of 
heat  occurring  in  the  vicinity  of  u'OO  deg.  ( '.  already  discovered  by  Roberts-Austen 
and  ascribed  by  him  to  the  presence  of  hydrogen,  were  also  observed  by  Carpenter 
and  Keeling,  as  well  as  some  faint  evolutions  in  the  vicinity  of  775  deg.,  the  meaning 
of  which  remains  uncertain. 


CHAPTER  XXVI  — EQUILIBKITM    DI.UIKAM    OF   IROX-CARBOX   ALLOYS     447 

When  it  became  apparent  that  graphite  and  not  cementite  must  he  the  final  stable 
form  of  carbon,  several  authorities  argued  that  two  equilibrium  conditions  could  exist 
according  to  the  rate  of  cooling  during  solidification,  one  of  them  stable,  the  other 
metiistable,  and  that  this  should  be  indicated  in  the  diagram.  This  view  was  presented 
notably  by  ('harpy  and  Grenet,  by  Benedicks  and  by  Heyn.  The  double  diagram 
advocated  by  them  is  represented  in  Figure  430.  The  solidification  of  free  cementite 
and  of  the  cementite-austenite  eutectic  being  assumed  to  be  due  to  the  well-known 
phenomenon  of  surfusion  or  undercooling,  the  corresponding  curves  are  arbitrarily 
outlined  at  temperatures  slightly  lower  than  those  pertaining  to  the  formation  of  free 


500°- 

a  1 

Carbon,    per    Cent,. 
Fig.  431.  — Rosenli.-iin's  equilibrium  diagram  (1911). 

graphite  and  of  graphite-austenite  eutectic.  The  author  has  already  shown  why,  in 
his  opinion,  the  graphite  curves  should  bo  left  out.  The  view  that  cementite  always 
forms  during  the  solidification  of  iron-carbon  alloys  but  that  being  unstable  it  is 
readily  dissociated  into  ferrite  and  graphite,  seems  to  be  better  supported  by  experi- 
mental evidences  and  more  consistent  with  practical  facts. 

Rosenhain  has  recently  plotted  the  experimental  results  of  Carpenter  and  Keel- 
ing, of  Gutowsky  and  of  himself,  obtaining  the  diagram  reproduced  in  Figure  431.  He 
considers  Gutowsky's  results  in  regard  to  the  form  of  the  solidus  curve  of  alloys  form- 
ing solid  solutions  as  more  accurate  than  those  previously  published,  arid  he  incorpo- 
rates them  in  the  diagram  as  shown  in  Fig.  431,  the  solidus  line  being  rounded  instead 
of  straight  as  heretofore  represented.  In  justification  of  his  course,  Rosenhain  writes : 

"  We  have  now  to  consider  the  curved  portion  of  the  'solidus,'  the  line  AD.    This 


448     CHAPTER  XXVI  —  EQUILIBRIUM    DIAGRAM   OF   IROX-CARBOX   ALLOYS 


represents  the  temperatures  at  which  the  alloys  have  just  completed  their  freezing 
process,  that  is,  have  just  become  completely  solid,  or,  conversely,  it  represents  the 
temperature  of  incipient  fusion  on  heating.  In  the  earlier  investigations,  and  even  in 
those  of  Messrs.  Carpenter  and  Keeling,  these  temperatures  were  obtained  by  esti- 
mating the  point  on  each  of  the  cooling-curves  where  the  heat-evolution  due  to  solidi- 
fication came  to  an  end.  Unfortunately,  the  end  of  a  heat-evolution  is  never  sharply 
indicated  on  the  curves,  so  that  this  estimation  was  admittedly  vague.  Quite  recently 
that  determination  has  been  repeated,  and  with  considerable  greater  accuracy,  lie- 
cause  a  very  much  more  satisfactory  method  was  available  .  .  . 

"The  method  of  determining  the  'solidus'  was  to  take  small  pieces  of  steel,  of 
known  composition,  heat  them,  and  suddenly  cool  them  from  successively  higher 
temperatures;  afterward  each  specimen  was  examined  by  means  of  the  microscope. 


IfiOO 

A 

^ 

*^* 

1200 

2 

f 

\3 

+  liquid 

^ 

liquid 

^ 

gra. 

phite  + 

iquid 

N, 

-A 

^ 

~v5--'^ 

D 

9 

^*^ 

r+grap 

TitC 

1145°-^ 

F 

H       7 

El 

H 

1095"/ 

G 

,915° 

/        J 

tFe.C 

o 

FecC 

•f  grap! 

ite 

\78S° 

I 

CO 

R 

^800° 

T 

K 

M 

or-tt  \ 

1 

r 

+  F< 

:3C 

X725" 

* 

Fe3C  - 

graphit 

e 

S 

a 

+  Fe,C 

S  S1 

,615° 

«> 

W 

600 

V 

u 

o 

0 

^y  ^.  jTgo 

C 

400 

Percent 

carbon 

N   a 
X    & 

123 

4              567891 

*  Fig.  432.  —  Upton's  equilibrium  diagram. 

It  is  easy,  as  the  photographs  show,  to  determine  what  is  the  particular  point  at  which 
you  have  reached  a  temperature  where  there  was  a  small  quantity  of  liquid  metal 
present  at  the  moment  of  quenching." 

Upton's  Diagram.  —  The  rather  revolutionary  diagram  shown  in  Figure  432  has 
been  proposed  by  G.  B.  Upton.  He  considers  it  more  satisfactory  than  the  double 
diagram,  as  it  does  away  with  the  necessary  conception  of  a  metastable  and  a  stable 
equilibrium  at  nearly  the  same  temperature.  Upton  argues  that  cementite  decom- 
poses above  800  deg.  C.  into  graphite  and  iron,  but  that  even  after  prolonged  an- 
nealing some  3.5  per  cent  carbon  remains  combined  from  which  it  is  inferred  that  the 
carbide  Fe6C  forms  (carbon  3.46  per  cent)  stable  in  the  region  LEFK  in  the  dia- 
gram although  he  adds  that  it  might  be  a  solid  solution  containing  that  amount  of 
carbon  which  forms  at  about  800  deg.  Fe6C  is  claimed  to  be  converted  into  Fe3C  thus 
accounting  for  the  evolutions  of  heat  in  this  region  reported  by  Carpenter  and  Keel- 
ing. At  about  600  deg.  according  to  the  diagram  Fe3C  is  converted  into  Fe=C  thus 
explaining  the  evolutions  of  heat  detected  at  that  temperature  by  many  observers. 
It  is  very  difficult  to  accept  the  hypothesis  that  graphite  is  not  the  stable  condition  of 


CHAPTER  XXVI  —  EQUILIBRIUM   DIAGRAM   OF  IRON-CARBON   ALLOYS     449 

carbon  in  the  area  LEHR  and  that  Fe2C  and  not  Fe3C  is  the  condition  of  the  carbon 
remaining  combined  after  complete  slow  cooling.  The  existence  of  both  FeeC  and 
Fe2C  is  based  on  chemical  tests  the  significance  of  which  is  far  from  evident.  Justi- 
fication for  the  line  EF  is  claimed  on  the  ground  that  evolutions  of  heat  have  been 
reported  by  Carpenter  and  Keeling  as  occurring  at  1050  to  1100  deg.  Other  ob- 
servers, however,  have  generally  failed  to  detect  these  retardations.  There  are 


2700 


2500  - 


2300' 


8100 


1900 


1700 


700 


0       S  1  2  3  4  5  6  7  3 

Fig.  433.  —  Ruff's  equilibrium  diagram. 


10 


other  serious  objections  to  Upton's  diagram  and  its  construction  is  so  highly  specula- 
tive as  to  render  its  closer  study  inadvisable  in  these  pages. 

Ruff's  Diagram.  — •  Professor  Ruff  has  published  the  equilibrium  diagram  repro- 
duced in  Figure  433,  which,  it  will  be  noted,  is  of  the  double  type,  GK  representing 
equilibrium  with  graphite  and  SK  with  cementite.  The  diagram  does  not  extend 
below  the  eutectoid  line  PK  but  includes  alloys  containing  as  much  as  10  per  cent 
carbon.  The  line  D'H  indicates  the  increasing  solubility  of  carbon  in  iron  with  in- 
creasing temperature.  At  some  2200  deg.  C.  molten  iron  saturated  with  carbon 
contains  nearly  10  per  cent  of  that  element.  At  this  temperature  Ruff  believes  that 
the  alloy  consists  of  the  carbide  Fe2C  as  explained  in  Chapter  XXII.  As  the  tem- 
perature increases  above  2200  deg.  the  solving  power  of  iron  for  carbon  decreases  as 


450     CHAPTER  XXVI —  EQUILIBRIUM    DIAGRAM  OF   IRON-CARBON   ALLOYS 

indicated  by  the  line  HI,  carbon  being  rejected,  probably  through  the  breaking  up 
of  some  Fe2C  into  iron  and  graphite  (see  Chapter  XXII).  At  some  2700  deg.  iron 
can  retain  but  a  little  over  7  per  cent  of  carbon  in  solution.  In  cooling  below  2200 
•deg.  (line  HD')  graphite  and  Fe3C  are  formed  from  the  transformation  of  Fe2C 
(3Fe2C  =  2  Fe3C  +  C). 


0         1         234        SO        1         8         9       10 

Fig.  434. — Wittorff's  equilibrium  diagram. 


Wittorff 's  Diagram.  —  N.  M.  Wittorff  worked  out  the  diagram  reproduced  in 
Figure  434.  It  will  be  noted  that  (1)  starting  with  an  alloy  containing  some  11  per 
cent  carbon,  as  it  cools  from  T  to  R,  that  is,  from  some  2400  to  2000  deg.  C.  the 
carbide  FeC2  crystallizes  out  according  to  the  diagram,  (2)  between  R  and  M  (2000 
to  1700  deg.)  Fe3C  forms  through  the  transformation  of  some  FeC2  (3)  between  M 
and  D  (1700  to  1350  deg.)  both  FeC  and  Fe4C  are  formed  necessarily  at  the  expense 
of  FeC2  which  disappears  and  possibly  also  through  the  transformation  of  some  of 
the  Fe3C,  and  (4)  below  D  (1350  deg.)  the  liquid  phase  disappears. 


CHAPTER  XXVI  —  EQUILIBRIUM    DIAGRAM   OF   IRON-CARBON   ALLOYS     451 

Investigations  of  a  region  of  the  equilibrium  diagram  but  little  explored  should 
be  welcomed  but  in  view  of  the  fact  that  the  experimental  results  of  the  different 
observers  as  well  as  the  conclusions  based  upon  them  are  at  such  variance,  it  seems 
advisable  to  omit  from  these  pages  a  closer  scrutiny  of  these  recent  diagrams.  When 
some  of  the  points  that  need  clearing  up  have  been  settled  the  matter  involved  will 
be  duly  incorporated  in  a  subsequent  edition  of  this  treatise. 


CHAPTER  XXVII 

THE  PHASE  RULE 

The  Phase  Rule  to  which  references  have  been  made  in  the  preceding  chapters 
should  now  be  considered  as  it  has  been  found  of  much  assistance  in  interpreting  cor- 
rectly the  iron-carbon  equilibrium  diagram. 

Enunciation  of  the  Phase  Rule.  —  The  phase  rule  was  enunciated  in  1878  by  Wil- 
lard  Gibbs,  at  the  time  Professor  of  Physics  in  Yale  University.  It  is  one  of  the  most 
notable  contributions  ever  made  to  physical  chemistry. 

The  phase  rule  deals  with  the  equilibrium  of  systems  and  is  generally  expressed 

by  the  formula: 

F  =  C  +  2-P 

showing  the  relation  existing  between  the  degrees  of  freedom  (F)  of  a  system,  the  num- 
ber of  components  (C),  and  the  number  of  phases  (P);  it  tells  us  that  the  number  of 
degrees  of  freedom  of  any  system  is  equal  to  the  number  of  its  components  plus  two, 
minus  the  number  of  phases  present.  In  order  to  understand  the  phase  rule  and  its 
application,  it  is  necessary  and  sufficient  to  have  an  accurate  understanding  of  the 
meaning  of  the  terms  employed  in  its  enunciation,  namely,  equilibrium,  degrees  of 
freedom,  components,  and  phases. 

Equilibrium.  —  A  substance  or  system  may  be  said  to  be  in  a  state  of  equilibrium 
when  it  is  chemically  and  physically  at  rest,  meaning  by  chemical  rest  that  chemical 
compounds  are  neither  being  dissociated  nor  formed,  and  by  physical  rest,  not  the 
absence  of  motion  but  the  absence  of  molecular  transformation,  such  as  changes  of 
state  or  allotropic  changes.  It  is  necessary,  however,  to  distinguish  between  homo- 
geneous and  heterogeneous  substances.  A  substance  is  said  to  be  homogeneous  when  it 
is  chemically  and  physically  uniform  throughout,  i.e.  when  any  two  portions  of  it  pos- 
sess identical  chemical  and  physical  properties.  Homogeneous  substances  are  neces- 
sarily gaseous  mixtures,  elements,  chemical  compounds,  or  liquid  and  solid  solutions. 
The  equilibrium  of  a  homogeneous  system  is  sometimes  called  homogeneous  equi- 
librium. A  heterogeneous  substance  is  made  up  of  two  or  more  physically  separate 
parts,  that  is,  of  parts  having  different  physical  properties.  Ice  and  water,  many 
rocks,  and  many  alloys  are  instances  of  heterogeneous  substances.  If  the  com- 
ponent parts  of  heterogeneous  substances  may  be  present  in  indefinite  proportions, 
the  substances  are  mechanical  mixtures;  if  they  occur  in  definite  proportions,  the 
substances  are  eutectic  or  eutectoid  alloys.  The  equilibrium  of  heterogeneous  systems 
is  sometimes  called  heterogeneous  equilibrium. 

Howe  has  recently  suggested  that  the  homogeneous  constituents  of  alloys  be  called 
"metarals"  because  of  the  great  analogy  between  the  constitution  of  metallic  alloys 
and  of  rocks,  the  minerals  being  the  homogeneous  components  of  the  latter,  while  the 
word  aggregate  is  very  frequently  used  to  designate  heterogeneous  alloys.  In  metal- 

452 


CHAPTER  XXVII  — THE   PHASE   RULE  453 

lography,  therefore,  metarals  and  aggregates  may  conveniently  replace  the  equivalent 
terms,  homogeneous  and  heterogeneous  substances,  of  the  physical  chemist. 

Only  three  independently  variable  factors  can  affect  the  equilibrium  of  a  system, 
namely,  (1)  the  temperature,  (2)  the  pressure,  and  (3)  the  concentration  or  composi- 
tion. If  arbitrary  values  may  be  given  to  one  or  more  of  these  factors  without  destroy- 
ing the  chemical  and  physical  rest  of  the  system,  its  equilibrium  is  said  to  be  stable. 
On  the  contrary,  if  a  change  in  value  of  any  one  of  these  three  factors  results  in  chemi- 
cal or  physical  transformation,  i.e.  in  atomic  or  molecular  activity  such  as  dissociation 
or  formation  of  chemical  compounds,  changes  of  state,  or  allotropic  changes,  the 
equilibrium  of  the  system  was  unstable.  Water  under  atmospheric  pressure  is  in 
stable  equilibrium,  for  we  may  change  its  temperature  within  wide  limits  without 
causing  it  to  undergo  a  change  of  state,  while  of  course  its  chemical  composition 
remains  likewise  unaffected.  All  elements  are  generally  in  a  state  of  stable  equilib- 
rium, as  well  as  an  infinite  number  of  substances  composed  of  two  or  more  elements, 
for  they  may  be  heated,  for  instance,  through  wide  ranges  of  temperatures  without 
upsetting  their  physico-chemical  equilibrium.  Examples  of  unstable  equilibrium, 
however,  are  far  from  uncommon.  During  the  solidification  of  substances,  for  in- 
stance, stages  must  generally  be  passed  through  during  which  the  equilibrium  of  the 
substance  is  unstable,  and  it  is  often  possible  through  very  rapid  cooling  to  retain 
in  the  cold  the  unstable  conditions,  because  of  the  rigidity  of  the  substance  now 
opposing  the  changes  needed  for  a  return  to  stable  equilibrium.  It  has  been  seen 
in  these  chapters  that  hardened  steel  is,  for  the  above  reason,  unstable,  hence  the 
possibility  of  tempering  it  by  slight  reheating. 

The  kind  of  equilibrium  known  as  metastable  remains  to  be  described.  Liquids 
may  be  cooled,  when  taking  the  necessary  precautions,  below  their  normal  freezing- 
point,  without  freezing,  the  phenomenon  being  known  as  superfusion,  surfusion,  or 
undercooling,  and  the  substance  being  said  to  be  in  metastable  equilibrium. 
Water,  for  instance,  may  be  cooled  below  0  deg.  C.  and  still  remain  liquid. 
The  introduction  of  a  solid  fragment  of  the  substance,  a  piece  of  ice  in  the  case 
of  water,  results  in  the  solidification  of  the  liquid  while  its  temperature  rises  to  its 
normal  freezing-point.  Otherwise,  the  substance  may  be  kept  liquid  below  its 
solidification  point  for  any  length  of  time.  If  the  temperature  of  the  liquid  con- 
tinues to  fall,  however,  a  point  is  reached  when  its  equilibrium  becomes  unstable,  i.e. 
when  further  lowering  of  temperature  causes  the  liquid  to  solidify.  To  state  the  case 
broadly,  the  failure  on  the  part  of  a  system  to  undergo  a  certain  chemical  or  physical 
transformation  when  that  transformation  is  due,  although  given  the  necessary  time, 
results  in  metastable  equilibrium,  while  its  failure  to  undergo  a  transformation  because 
of  the  necessary  time  being  denied,  as  in  quenching,  results  in  unstable  equilibrium. 
Metastable  equilibrium  is  stable,  at  least  within  narrow  limits  of  temperature,  while, 
theoretically  at  least,  slight  heating  of  a  substance  in  unstable  equilibrium  should 
result  in  a  partial  return  to  a  more  stable  condition,  that  is,  in  a  partial  occurrence  of 
the  transformation  that  was  suppressed  by  quick  cooling. 

Degrees  of  Freedom.  —  By  the  degrees  of  freedom  (sometimes  called  degrees  of 
liberty),  of  a  system  is  meant  the  number  of  the  three  independently  variable  factors, 
temperature,  pressure,  and  concentration,  which  may  arbitrarily  be  made  to  vary 
without  disturbing  the  system's  physico-chemical  rest.  It  has  already  been  noted  that 
a  system,  in  order  to  be  in  stable  equilibrium,  must  have  at  least  one  degree  of  free- 
dom. It  will  also  be  understood  that  no  system  can  have  more  than  two  degrees  of 
freedom  because  in  the  case  of  arbitrary  values  being  given  to  two  of  the  factors,  the 


454  CHAPTER   XXVII —  THE    I'll  ASK    Kl'LK 

value  of  the  third  is  necessarily  fixed,  this  being  due  to  the  known  rigid  relations 
existing  between  temperature,  pressure,  and  concentration. 

Systems  which  have  no  degree  of  freedom  are  said  to  be  "unvariant"  or  "non- 
variant."  Their  equilibrium  is  necessarily  unstable.  Systems  having  one  degree  of 
freedom  are  called  "univariant"  or  "monovariant,"  and  those  with  two  degrees  of 
freedom  "bivariant"  or  "divariant." 

Phases.  —  By  the  phases  of  a  system  are  meant  the  homogeneous,  physically  dis- 
tinguishable, and  mechanically  separable  constituents  of  that  system.  Water  and  ice, 
for  instance,  are  possible  phases  of  the  water-ice  system;  quartz,  felspar,  and  mica  are 
phases  of  granite,  that  is,  of  the  silica-alumina-potash  system.  It  will  be  apparent  that 
phases  must  necessarily  be  gaseous  mixtures,  elements,  definite  chemical  compounds, 
or  solutions.  As  previously  mentioned,  Howe,  following  the  petrographical  nomencla- 
ture, and  noting  that  the  minerals  are  the  phases  of  rocks,  calls  "metarals"  the  phases 
of  metals  and  alloys. 

Components.  —  The  components  of  a  system  are  described  by  Findlay  as  "those 
constituents  the  concentration  of  which  can  undergo  independent  variation  in  the 
different  phases,"  by  Bancroft  as  "substances  of  independently  variable  concentra- 
tion," by  Mellor  as  those  "entities  which  are  undecomposable  under  the  conditions 
of  experiments,"  by  Howe  as  "free  elements  and  those  compounds  which  in  the  nature 
of  the  case  are  undecomposable  under  the  conditions  contemplated,  and  so  play  the 
part  of  elements."  The  components  of  a  system  may  be  either  chemical  compounds 
or  elements,  but  there  is  at  times  some  difficulty  in  grasping  the  distinction  between 
the  components  of  a  system  and  its  ultimate  chemical  constituents.  The  criterion  by 
which  to  decide  whether  an  entity  is  or  is  not  a  component,  is  the  possibility  of  in- 
dependent variation  in  the  different  phases.  Take  the  system  water,  for  instance: 
evidently  water  and  not  hydrogen  and  oxygen  is  the  component,  because  any 
variation  in  the  proportion  of  hydrogen  would  necessarily  imply  a  corresponding 
and  well-defined  variation  in  the  proportion  of  oxygen  and  vice  versa.  Findlay 
writes : 

"In  deciding  the  number  of  components  in  any  given  system,  not  only  must  the 
constituents  chosen  be  capable  of  independent  variation,  but  a  further  restriction  is 
imposed,  and  we  obtain  the  following  rule:  As  the  components  of  a  system  there  are  to  be 
chosen  the  smallest  number  of  independently  variable  constituents  by  means  of  which 
the  composition  of  each  phase  participating  in  the  state  of  equilibrium  can  be  expressed 
in  the  form  of  a  chemical  equation." 

In  the  case  of  alloys,  however,  such  difficulty  does  not  arise,  for  the  constituent 
metals  are  always  the  components  of  the  systems. 

The  Phase  Rule  Applied  to  Alloys.  —  In  dealing  with  alloys  we  may  for  all  practi- 
cal purposes  ignore  the  influence  of  pressure,  seeing  that  because  of  their  feeble  vola- 
tility they  are  practically  always  subjected  to  atmospheric  pressure.  Omitting  the 
influence  of  pressure  necessarily  reduces  by  one  the  possible  number  of  degrees  of 
freedom  so  that  in  the  case  of  alloys  the  phase  rule  may  be  expressed  by  the  formula : 

F  =  C  +  1 - P 

signifying  that  the  number  of  degrees  of  freedom  is  equal  to  the  number  of  components 
plus  one,  minus  the  number  of  phases.  Since  to  be  in  stable  equilibrium  a  system  must 
have  at  least  one  degree  of  freedom,  it  is  obvious  that  an  alloy  made  up  of  n  metals 
cannot  have  more  than  n  phases.  If  it  had  n  +  1  phases  it  would  have  no  degree  of 
freedom,  that  is,  its  equilibrium  would  be  unstable.  With  n  -  1  phases  it  would  have 


CHAPTER   XXVII  — THE   PHASE    RTLE 


455 


two  degrees  of  freedom.    It  could  not  have  less  than  n  —  1  phases,  since  it  cannot  have 
more  than  two  degrees  of  freedom. 

The  Phase  Rule  Applied  to  Pure  Metals.  —  Pure  metals  have  only  one  component, 
hut  their  possible  phases  are  (1)  liquid  metal,  (2)  solid  metal,  (3)  several  allotropic  con- 
ditions of  the  solid  metal.  Let  us  consider  Figure  435,  which  represents  the  solidi- 
fication of  a  pure  metal  as  explained  in  Chapter  XXV. 

Above  the  temperature  T  the  metal  is  entirely  liquid;  it  has  but  one  phase,  and 
consequently  one  degree  of  freedom  (F  =1  +  1  —  1  =  1).  The  system  above  T  is 
univariant;  its  temperature  may  be  altered  within  wide  limits-^vjthout  disturbing  its 


I 

^  T 


/  /me 

Fig.  4:ir>.  —  Equilibrium  of  pun;  metals  according  to  the  Phase  Rule. 

equilibrium:  it  remains  liquid.  At  the  temperature  T  two  phases  are  present,  solid 
metal  and  liquid  metal,  the  metal  having,  therefore,  no  degree  of  freedom  (F  =  1  +  1  — 
2  =  0):  it  is  non-variant.  Liquid  and  solid  metal  can  exist  only  at  one  temperature, 
the  critical  temperature  of  solidification,  any  change  of  its  temperature  resulting  in 
the  disappearance  of  one  of  the  phases,  that  is,  in  a  return  to  stable  equilibrium. 
Increasing  the  temperature  must  result  in  the  disappearance  of  the  solid  phase,  while 
lowering  the  temperature  must  cause  the  disappearance  of  the  liquid  phase.  Below 
the  temperature  T  the  system  contains  only  the  solid  phase,  being,  therefore,  univari- 
ant: its  temperature  may  be  varied  arbitrarily. 

The  Phase  Rule  Applied  to  Binary  Alloys.  —  Binary  alloys  having  for  components 
the  two  alloying  metals,  the  formula  of  the  phase  rule  becomes: 


F  =  2+ 1  - P 
or  F  =  3  -  P 


456 


CHAPTER   XXVII  — THE   PHASE   RULE 


Clearly  binary  alloys  when  in  a  condition  of  stable  equilibrium  cannot  have  more  than 
two  phases.  With  one  phase  they  will  be  bivariant,  with  two  phases  univariant,  and 
with  three  phases  non-variant.  Let  us  apply  the  rule  to  the  fusibility  curves  of  binary 
alloys  of  metals  partially  soluble  in  each  other  when  solid  (Fig.  436).  Above  the  liqui- 
dus  MEM'  there  is  but  one  phase  present,  namely  the  liquid  phase,  the  system  being, 
therefore,  bivariant  (F  =  3  -  1  =  2),  i.e.  both  temperature  and  concentration  may  be 
varied  arbitrarily  without  upsetting  the  equilibrium  of  the  system,  which  means,  in 
the  case  under  consideration,  without  causing  its  solidification.  On  reaching  any 
point  L  of  the  liquidus  the  alloy  begins  to  solidify,  and  two  phases  are  now  present, 
namely,  solid  solution  and  liquid  alloy,  the  system  becomes  univariant  (F  =  3  -  2  =  1). 
Having  but  one  degree  of  freedom  only  the  temperature  or  the  concentration  may  be 


Fig.  436.  —  Equilibrium  according  to  the  Phase  Rule  of  binary  alloys  whose  component  mchils  .-ire 
partially  soluble  in  each  other  in  the  solid  state. 

arbitrarily  varied.  Should  we,  for  instance,  lower  the  temperature  of  alloy  R  from  T 
to  T'  (Fig.  436)  the  composition  of  the  liquid  phase  necessarily  shifts  from  L  to  L',  and 
that  of  the  solid  phase  in  equilibrium  with  it  from  s  to  s'.  In  the  region  MSES'M'  of 
the  diagram  bounded  by  the  liquidus  and  solidus  lines,  therefore,  the  alloys  are  uni- 
variant, any  arbitrary  change  of  temperature  resulting  in  a  well-defined  change  of 
concentration  and  vice  versa.  At  E,  corresponding  to  eutectic  composition  and 
eutectic  temperature,  three  phases  are  present,  namely  two  solid  solutions  and  liquid 
alloy,  the  system  having  no  degree  of  freedom  (F  =  3  —  3  =  0) .  Neither  the  tempera- 
ture nor  the  concentration  may  be  altered  without  causing  the  disappearance  of  at 
least  one  of  the  phases.  Increasing  the  temperature  must  result  in  the  disappear- 
ance of  both  solid  solutions,  the  system  becoming  bivariant,  while  lowering  it  must 
be  followed  by  the  disappearance  of  the  liquid  phase.  Again,  shifting  the  concen- 
tration to  the  left  or  right  of  E  must  yield  the  univariant  system  solid  solution  plus 
•  liquid.  Clearly  two  solid  phases  and  a  liquid  phase  can  only  exist  at  one  critical 
temperature  and  for  one  critical  composition  of  the  alloy;  in  the  case  of  a  eutectic 


CHAPTER  XXVII  — THE   PHASE   RULE  457 

alloy  these  three  phases  can  exist  only  at  its  freezing  temperature.  In  the  areas 
AMSB  and  DM'S'C  single  homogeneous  solid  solutions  only  are  present,  that  is,  but 
one  phase  exists,  and  the  corresponding  alloys  have,  therefore,  two  degrees  of  freedom. 
Arbitrary  changes  both  of  temperature  and  composition  within  these  areas  do  not 
disturb  the  equilibrium  of  the  system.  Within  the  region  BSS'C  two  phases  occur, 
solid  solution  M  and  solid  solution  M',  the  corresponding  alloys  having,  therefore,  but 
one  degree  of  freedom.  Increasing  the  temperature  from  P  to  P',  for  instance,  must 
result  in  shifting  the  composition  of  the  solid  solutions  respectively  from  R  to  R'  and 
from  0  to  0'. 

The  Phase  Rule  Applied  to  Iron-Carbon  Alloys.  —  Since  iron-carbon  alloys  belong 
to  the  class  of  binary  alloys  the  constituents  of  which  are  partially  soluble  in  each 
other  in  the  cold,  the  application  of  the  phase  rule  to  their  equilibrium  diagram  should 
•  not  present  any  difficulty,  but  we  have  now  to  consider  allotropic  changes  as  well  as 
changes  of  state.  Their  possible  phases  or  metarals  are:  (1)  liquid  iron,  (2)  liquid 
solution  of  carbon  (or  FesC)  in  iron,  (3)  solid  solution  (austenite)  of  carbon  (or  FesC) 
in  gamma  iron,  (4)  solid  gamma  iron,  (5)  solid  beta  iron,  (6)  solid  alpha  iron  (ferrite), 
(7)  solid  solution  (martensite) 1  of  carbon  (or  FesC)  in  beta  iron,  (8)  solid  cementite, 
(9)  graphite,  and  possibly  others.  The  exact  nature  of  troostite  and  sorbite  being 
still  in  doubt,  they  are  not  here  classified  as  phases,  seeing  that  they  may  be,  and 
probably  are,  aggregates  of  two  or  more  phases,  unless  indeed  they  be,  according  to 
Benedicks,  emulsions  or  colloidal  solutions.  Scientists  do  not  agree,  however,  as  to 
whether  colloidal  solutions  are  or  are  not  phases,  opinions  differing  in  regard  to  their 
homogeneity.  Indeed  some  writers  like  Le  Chatelier  question  the  existence  of  colloidal 
solutions  which  they  consider  as  finely  divided  aggregates.  Pearlite  evidently  is  not 
a  phase,  but  an  aggregate  of  the  two  phases,  ferrite  and  cementite,  in  constant  pro- 
portion after  the  fashion  of  eutectic  and  eutectoid  mixtures. 

Let  us  now  apply  the  teachings  of  the  phase  rule  to  the  iron-carbon  equilibrium 
diagram  (Fig.  437) .  The  number  and  kinds  of  phases  existing  at  different  temperatures, 
and  for  different  proportions  of  the  components,  iron  and  carbon,  have  been  clearly 
indicated  and  will  be  readily  understood  in  view  of  the  foregoing  considerations. 
Above  the  liquidus  LEU  all  alloys^  are  composed  of  but  one  liquid  phase,  and  have, 
therefore,  two  degrees  of  freedom;  between  the  liquidus  and  solidus,  that  is,  in  the  region 
LSE  and  L'S'E,  two  phases  are  present,  liquid  solution  plus  solid  solution  (austenite), 
or  liquid  solution  plus  solid  FesC,  hence  the  corresponding  alloys  have  here  but  one 
degree  of  freedom^ (alloys  of  composition  E  and  at  the  corresponding  temperature  are 
evidently  made  up  of  three  phases,  namely  liquid  solution  plus  solid  austenite  plus 
solid  cementite,  being,  therefore,  non-variant;  in  the  region  LADS  all  alloys  being  com- 
posed of  but  one  phase,  namely,  solid  austenite,  are  bivariant;  at  D  the  alloy  contains 
three  phases,  ferrite,  cementite,  and  austenite,  and  is,  therefore,  non- variant;  in  the 
area  DSS'F  two  phases  are  present,  solid  solution  (austenite)  plus  cementite,  and  the 
system  has  but  one  degree  of  freedom.  If,  as  is  often  the  case,  cementite  is  in  this 
region  decomposed  into  iron  and  graphite  the  alloys  are  for  the  time  being  non-variant, 
becoming  again  univariant  on  the  complete  disappearance  of  cementite.  In  the  region 
ABH  beta  iron  and  austenite  are  present,  the  alloys  having  in  consequence  but  one 
degree  of  freedom;  in  the  region  BCDH  alpha  ferrite  and  austenite  are  present  and 
the  alloys,  therefore,  are  univariant.  Finally  below  CDF  three  possible  cases  should 
be  considered:  (I)  the  cementite  formed  during  solidification  and  subsequent  cooling 

1  All  investigators  do  not,  agree  as  to  the  homogeneity,  that  is  the  phase-like  character  of  marten- 
site,  some  still  regarding  it  as  an  aggregate. 


458 


CHAPTER  XXVII  — THE   Pi  [ASK   RULE 


<0 


CHAPTER   XXVII  — THE   PHASE   RULE  459 

remains  unchanged,  in  which  case  the  alloys  are  made  up  of  the  two  phases  ferrite  and 
cementite,  being,  therefore,  univariant,  their  equilibrium,  however,  as  previously  ex- 
plained, is  supposed  to  be  metastable;  (II)  the  cementite  has  been  completely  con- 
verted into  ferrite  and  graphite,  only  those  two  phases  being  present,  undoubtedly 
representing  the  stable  equilibrium  of  all  iron-carbon  alloys;  (III)  the  dissociation  of 
cementite  has  been  incomplete,  both  cementite  and  graphite  being  present,  which 
with  ferrite  give  three  phases,  the  corresponding  alloys  being  non-variant  and,  there- 
fore, their  equilibrium  unstable.  Condition  (I)  generally  prevails  in  all  grades  of  steel, 
and  is  readily  produced  in  cast  iron  by  rapid  cooling  especially Jnjthe  absence  of  con- 
siderable silicon,  the  resulting  alloys  being  known  as  white  cast  iron.  Condition  (II) 
never  obtains  in  steel,  but  may  be  produced  in  highly  carburized  alloys  by  very  slow 
cooling  through  and  below  solidification,  especially  in  the  presence  of  much  silicon 
and  in  the  absence  of  manganese  and  sulphur.  Condition  (III)  is  the  condition  of  the 
gray  cast  irons  of  commerce,  their  compositions  and  other  influences  prevailing  during 
their  cooling  being  such  as  to  cause  the  graphitizing  of  varying  proportions  of  cementite. 


APPENDIX 

REPORT    OF    COMMITTEE    53    OF    THE    INTERNATIONAL    ASSOCIATION    FOR 

TESTING   MATERIALS 

ON   THE  NOMENCLATURE  OF  THE  MICROSCOPIC  SUBSTANCES 
AND  STRUCTURES  OF  STEEL  AND   CAST  IRON 

Presented  by  the  Chairman  H.  M.  HOWE  and  the  Secretary  of  the  Committee  ALBERT  SAUVEUR 
at  the  VIth  Congress,  New  York,  September,  1912 

The  Committee  for  studying  this  problem  is  constituted  as  follows: 

Professor  H.  M.  HOWE,  Chairman,  New  York. 

Professor  ALBERT  SAUVEUR,  Secretary,  Cambridge,  Mass. 

Members:  F.  OSMOND,  Paris;  Dr.  H.  C.  H.  CARPENTER,  Manchester;  Prof.  W. 
CAMPBELL,  New  York;  Prof.  C.  BENEDTCKS,  Stockholm;  Prof.  F.  WUST,  Aachen; 
Prof.  A.  STANSFIELD,  Montreal;  Dr.  J.  E.  STEAD,  Middlesbrough;  Prof.  L.  GUILLET, 
Paris;  Prof.  E.  HEYN,  Berlin-Lichterfelde;  Dr.  W.  ROSENHAIN,  Teddington. 

I.     GENERAL    PLAN 

We  first  enumerate  the  substances  of  such  importance  as  to  warrant  it,  indicating 
roughly  their  constitution,  and  then  define  and  describe  certain  of  them. 

The  conditions  which  we  meet  are  (1)  that  we  need  definitions  on  which  all  can 
agree;  and  this  implies  that  they  must  be  free  from  all  contentious  matter  and  be 
based  on  what  all  admit  to  be  true;  (2)  that  the  reader  must  needs  know  the  current 
theories  as  to  the  constitution  of  these  substances,  and  these  theories  are  necessarily 
contentious.  We  meet  these  conditions  by  the  plan  of  giving  (1)  the  Name  which  we 
recommend  for  general  use,  followed  immediately  in  parentheses  by  the  other  names 
used  widely  enough  to  justify  recording  them;  (2)  the  Definition  proper,  based  on  an 
undisputed  quality,  e.g.  that  of  austenite,  which  we  base  on  its  being  an  iron-carbon 
solid  solution,  purposely  omitting  all  reference  to  the  precise  nature  of  solvent  and 
solute;  and  (3)  Constitution,  etc.,  etc.,  in  which  we  give  the  current  theories  as  to  the 
nature  of  solvent  and  solute  and  appropriate  descriptive  matter. 

The  distinction  between  these  three  parts  should  be  understood.  (1)  The  Names 
actually  used  are  matters  of  record  and  indisputable.  (2)  The  Definitions  are  matters 
of  convention  or  treaty,  binding  on  the  contracting  parties,  though  subject  to  de- 
nouncement, preferably  based  on  some  determinable  property  of  the  thing  defined  as 
distinguished  from  any  theory  as  to  its  nature,  or  if  necessarily  based  on  any  theory 
it  should  be  a  theory  which  is  universally  accepted.  It  is  a  matter  purely  of  conven- 
tion and  general  convenience  what  individual  property  of  the  thing  defined  shall 
form  the  basis  of  the  definition.  The  name  and  the  definition  should  endure  perma- 
nently, except  in  the  case  of  a  definition  based  on  an  accepted  theory,  which  must  be 

460 


APPENDIX  —  NOMENCLATURE  OF  THE  MICROSCOPIC  CONSTITUENTS     461 

changed  if  the  theory  should  later  be  disproved.  (3)  Theories  and  Descriptions  are 
not  matters  of  agreement  or  convention  but  dependent  on  observation,  and  therefore 
always  subject  to  be  changed  by  new  discoveries.  They  are  temporary  in  their  nature 
as  distinguished  from  the  names  and  definitions  which  should  be  fixed,  at  least  rela- 
tively. 

This  case  of  austenite  illustrates  the  advantage  of  non-indicative  names.  The 
names  which  we  propose  to  displace,  "gamma  iron"  and  "mixed  crystals,"  imply 
definite  theories  as  to  the  nature  of  austenite,  and  hence  might  have  to  be  abandoned 
in  case  those  theories  were  later  disproved.  The  name  "austenite"  implies  nothing, 
like  mineralogical  names  in  general,  and  hence  is  stable  in  itself.  Our  infant  branch 
of  science  may  well  learn  from  its  elder  sister,  which  has  tried  and  proved  the  advan- 
tage of  this  non-indicative  naming. 

In  those  cases  in  which  a  name  has  been  used  in  more  than  one  sense  we  advise 
the  retention  of  one  and  the  abandonment  of  the  others,  having  obtained  the  consent 
of  the  proposers  of  such  names  for  their  abandonment. 

Many  whose  judgment  we  respect  object  to  our  including  certain  of  the  less  used 
names,  e.g.  from  i  to  n  in  our  list,  holding  them  either  to  be  confusing  or  to  be  needless. 

It  is  true  that  several  names  (hardenite,  martensite,  sorbite,  etc.),  have  been  used 
with  various  meanings,  and  hence  confusingly,  in  spite  of  which  most  of  them  should 
be  retained,  each  with  a  single  sharp-cut  definition,  because  they  are  so  useful. 

As  regards  the  alleged  needlessness  of  certain  names  it  is  for  each  writer  to  decide 
whether  he  does  or  does  not  need  names  with  nice  shades  of  meaning,  such  as  osmon- 
dite  and  troosto-sorbite.  Those  who  look  only  at  the  general  outlines  and  not  at  the 
details  have  no  right  to  forbid  the  workers  in  detail  from  having  and  using  words 
fitting  their  work;  nor  have  those  whose  needs  are  satisfied  by  the  three  primary 
colors  a  right  to  forbid  painters,  dyers,  weavers,  and  others  from  naming  the  many 
shades  with  which  they  are  concerned.  Like  the  lexicographer  we  must  serve  the 
reader  by  explaining  those  words  which  he  will  meet,  whether  we  individually  use  or 
condemn  them.  We  feel  that  we  have  exhausted  our  powers  in  cautioning  writers 
that  certain  words  are  rare  and  not  likely  to  be  understood  by  most  readers,  or  are 
improper  for  any  reason,  and  in  urging  the  complete  abandonment  of  those  with- 
drawn by  their  proposers. 

Needless  words  will  die  a  natural  death;  needed  ones  we  cannot  kill.  The  good 
we  might  do  in  hastening  the  death  of  the  moribund  by  omitting  them  from  this  re- 
port is  less  than  the  good  we  do  by  teaching  their  meaning  to  those  who  will  meet 
them  in  ante-mortem  print.  These  readers  have  rights.  We  serve  no  class,  but  the 
whole. 

Illustrations.  —  At  the  end  of  the  several  descriptions  the  reader  is  referred  to 
good  illustrations  in  Osmond  and  Stead's  "Microscopic  Analysis  of  Metals,"  Griffin 
&  Co.,  London,  1904. 

II.    LIST   OF    MICROSCOPIC   SUBSTANCES 

The  microscopic  substances  here  described  consist  of 

1.  Meiarals,  true  phases,  like  the  minerals  of  nature.  These  are  either  elements, 
definite  chemical  compounds,  or  solid  solutions  and  hence  consisting  of  definite  sub- 
stances in  varying  proportions.  These  include  austenite,  ferrite,  cementite,  and 
graphite. 


462    APPENDIX  —  NOMENCLATURE  OF  THK  MICROSCOPIC  CONSTITUENTS 

2.  Aggregates,  like  the  petrographic  entities  as  distinguished  from  the  true  minerals. 
These  mixtures  may  he  in  definite  proportions,  i.e.  eutectic,  -or  eutectoid  mixtures 
(ledeburite,  pearlite,  steadite),  or  in  indefinite  proportions  (troostite,  sorbite).  Those 
aggregates  which  are  important  for  any  reason  are  here  described. 

(Many  true  minerals,  such  as  mica,  felspar,  and  hornblende,  are  divisible  into 
several  different  species  so  that  these  true  mineral  names  may  be  either  generic  or 
specific.  These  genera  and  species  are  definite  chemical  compounds  in  which  one 
element  may  replace  another.  Other  minerals,  such  as  obsidian,  are  solid  solutions 
in  varying  proportions,  and  in  these  also  one  element  may  replace  another.  Metarals 
like  minerals  differ  from  aggregates  in  being  severally  chemically  homogeneous.) 

These  two  classes  may  be  cross  classified  into : 

(A)  The  iron-carbon  series,  which  come  into  being  in  cooling  and  heating. 

(B)  The  important  impurities,  manganese  sulphide,  ferrous  sulphide,  slag,  etc. 

(C)  Other  substances. 

The  most  prominent  members  of  the  iron-carbon  series  are: 

I.  molten  iron,  metaral,  molten  solution,  but  hardly  a  microscopic  constituent; 

II.  the  components  which  form  in  its  solidification : 

(a)  austenite,  solid  solution  of  carbon  or  iron  carbide  in  iron,  metaral, 
(6)  cementite,  definite  metaral,  Fe3C, 

(c)  graphite,  definite  metaral,  C; 

III.  the  transition  substances  which  form  through  the  transformation  of  austenite 
during  cooling: 

(d)  martensite,  metaral  of  variable  constitution;  its  nature  is  in  dispute; 

(e)  troostite,  indefinite  aggregate,  uncoagulated  mixture, 

(/)  sorbite,  indefinite  aggregate,  chiefly  uncoagulated  pearlite  plus  ferrite  or  ce- 
mentite; 

IV.  products1  of  the  transformation  of  austenite: 
(g)  ferrite, 

(/*)  pearlite. 

This  transformation  may  also  yield  cementite  and  graphite  as  end  products  in 
addition  to  those  under  6  and  c. 

In  addition  to  the  above,  the  names  of  which  are  universally  recognized  and  in 
general  use,  the  following  names  have  been  used  more  or  less: 

(i)  ledeburite  (Wiist),  definite  aggregate,  the  austenite-cementite  eutectic; 

(j)  ferronite  (Benedicks),  hypothetical  definite  metaral,  ft  iron  containing  about 
0.27  per  cent  of  carbon; 

(k)  steadite  (Sauveur) ,  definite  aggregate,  the  iron-phosphorus  eutectic  (rare)  ; 

and  three  transition  stages  in  the  transformation  of  austenite,  viz. : 

(I)  hardenite  (Arnold),  collective  name  for  the  austenite  and  martensite  of  eutec- 
toid composition; 

(m)  osmondite  (Heyn),  boundary  stage  between  troostite  and  sorbite; 

(ri)  troosto-sorbite  (Kourbatoff ) ,  indefinite  aggregate,  the  troostite  and  the  sorbite 
which  lie  near  the  boundary  which  separates  these  two  aggregates  (obsolescent) . 

'In  hypo-eutectoid  steels  these  habitually  play  the  part  of  end  products,  though  according  to 
the  belief  of  most  the  true  end  of  the  transformation  is  not  reached  till  the  whole  has  changed  into 
a  conglomerate  of  ferrite  plus  graphite. 


APPENDIX  —  NOMENCLATURE  OF  THE  MICROSCOPIC  CONSTITUENTS     463 


III.     DEFINITIONS  AND   DESCRIPTIONS 

Carbon-Iron  Equilibrium  Diagram,  Figure  438.  —  Under  the  several  substances 
about  to  be  described  an  indication  will  be  given  of  the  parts  of  the  carbon-iron  equi- 
librium diagram  Figure  438  to  which  they  severally  correspond. 

Austenite,  Osmond  (Fr.  Austenite,  Ger.  Austenit,  called  also  mixed  crystals  and 
gamma  iron.  Up  to  the  year  1900  often  called  martensite  and  wrongly  sometimes 
still  so  called).  Metaral  of  variable  composition. 

Definition.  —  The  iron-carbon  solid  solution  as  it  exists  above  the  transformation 


1500 
1400 
1300 
1200 


A 


iooo- 

rj 
I* 

o 

B  900- 

o 

H 

800- 
M 

700- 
600- 


500- 


1. 
Molten    Iron 

(Per  Fondu) 


Molten 

(Per  Fondu) 

•f 

flusbenibe 


C 
/ 

3. 


/Austenite 


5. 

Austenite  +  Cementite 


Pearl  ite 


8.B. 

Cementite 

Pearl  ite 


K 


1  2  3  4  5 

Carbon  per  cent 

Y\K.  438. —  A,:  The  line  PSK  is  often  called  "Ai".    A3:  The  line  COS  is  often  called  "A,",  and 
this  name  is  sometimes  applied  to  the  line  SE. 

range  or  as  preserved  with  but  moderate  transformation  at  lower  temperatures,  e.g. 
by  rapid  cooling,  or  by  the  presence  of  retarding  elements  (Mn,  Ni,  etc.),  as  in  12  per 
cent  manganese  steel  and  25  per  cent  nickel  steel. 

Constitution  and  Composition.  —  A  solid  solution  of  carbon  or  iron  carbide  (prob- 
ably Fe3C)  and  gamma  iron,  normally  stable  only  above  the  line  PSK  of  the  carbon- 
iron  diagram.  It  may  have  any  carbon  content  up  to  saturation  as  shown  by  the  line 
SE,  viz.:  about  0.90  per  cent  at  S  (about  725  deg.  C.)  to  1.7  per  cent  at  E  (about 
1130  deg.).  The  theory  that  the  iron  and  the  carbide  or  carbon,  instead  of  being  dis- 
solved in  each  other,  are  dissolved  in  a  third  substance,  the  solution  of  eutectoid  com- 


464     APPENDIX  —  NOMENCLATURE  OF  THE  MICROSCOPIC  CONSTITUENTS 

position  (Fe24C,  called  hardenite)  is  not  in  accord  with  the  generally  accepted  theory 
of  the  constitution  of  solutions,  and  is  not  entertained  widely  or  by  any  member  of 
this  committee. 

Crystallization.  —  Isometric.  The  idiomorphic  vug  crystals  are  octahedra  much 
elongated  by  parallel  growth.  The  etched  sections  show  much  twinning.  (Osmond 
and  most  authorities.)  Le  Chatelier  believes  it  to  be  rhombohedral.  Cleavage 
octahedral. 

Varieties  and  Genesis. —  (l)  Primary  austenite  formed  in  the  solidification  of  carbon 
steel  and  hypo-eutectic  cast  iron;  (2)  eutectic  austenite,  interstratified  with  eutectic 
cementite,  making  up  the  eutectic  formed  at  the  end  of  the  solidification  of  steel  con- 
taining more  than  about  1.7  per  cent  of  carbon,  and  of  all  cast  iron. 

Equilibrium.  —  It  is  normal  and  in  equilibrium  in  Region  4,  and  also  associated 
with  beta  iron  in  Region  6,  with  a  iron  in  Region  7,  and  with  cementite  in  Region  5. 
It  should  normally  transform  into  pearlite  with  either  ferrite  or  cementite  on  cooling 
past  AI  into  Region  8. 

Transformation.  —  In  cooling  slowly  through  the  transformation  range,  Ar3  -  Ari, 
austenite  shifts  its  carbon  content  spontaneously  through  generating  pro-eutectoid 
ferrite  or  cementite,  to  the  eutectoid  ratio,  about  0.90  per  cent,  and  then  transforms 
with  increase  of  volume  at  Ari  into  pearlite,  q.v.,  with  which  the  ejected  ferrite  or 
cementite  remains  mixed.  Rapid  cooling  and  the  presence  of  carbon,  manganese,  and 
nickel  obstruct  this  transformation,  (1)  retarding  it,  and  (2)  lowering  the  temperature  at 
which  it  actually  occurs,  and  in  addition  (3)  manganese  and  nickel  lower  the  temperature 
at  which  in  equilibrium  it  is  due.  Hence,  by  combining  these  four  obstructing  agents 
in  proper  proportions  the  transformation  may  be  arrested  at  any  of  the  intermediate 
stages,  martensite,  troostite,  or  sorbite,1  q.v.,  and  if  arrested  in  an  earlier  stage  it 
can  be  brought  to  any  later  desired  stage  by  a  regulated  reheating  or  "tempering." 
For  instance,  though  a  very  rapid  cooling  in  the  absence  of  the  three  obstructing  ele- 
ments checks  the  transformation  but  little  and  only  temporarily,  yet  if  aided  by  the 
presence  of  a  little  carbon  it  arrests  the  transformation  wholly  in  the  martensite 
stage;  and  in  the  presence  of  about  1.50  per  cent  of  carbon  such  cooling  retains  about 
half  the  austenite  so  little  altered  that  it  is  "considerably"  softer  than  the  usually 
darker  needles  of  the  surrounding  martensite,  with  which  it  contrasts  sharply.  Again, 
either  (a)  about  12  per  cent  of  manganese  plus  1  per  cent  of  carbon,  or  (6)  25  per  cent 
of  nickel,  lower  and  obstruct  the  transformation  to  such  a  degree  that  austenite  per- 
sists in  the  cold  apparently  unaltered,  even  through  a  slow  cooling.  (Hadfield's  man- 
ganese steel  and  25  per  cent  nickel  steel,  manganiferous  and  nickeliferous  austenite 
respectively.) 

Occurrence.  —  When  alone  (12  per  cent  manganese  and  25  per  cent  nickel  steel 
and  Maurer's  2  per  cent  carbon  plus  2  per  cent  manganese  austenite)  polyhedra,  often 
coarse,  much  twinned  at  least  in  the  presence  of  martensite,  and  readily  developing 
slip  bands.  In  hardened  high-carbon  steel  it  forms  a  ground  mass  pierced  by  zigzag 
needles  and  lances  of  martensite. 

Etching.  —  All  the  common  reagents  darken  it  much  more  than  cementite,  less 

1  Though  the  transformation  can  be  arrested  in  such  a  way  as  to  leave  the  whole  of  the  steel 
in  the  condition  of  martensite,  it  is  doubted  by  some  whether  it  can  be  so  arrested  as  to  leave  the 
whole  of  it  in  any  of  the  other  transition  stages.  Troostite  and  sorbite  caused  by  such  arrest  are 
habitually  mixed,  troostite  with  martensite  or  sorbite  or  both,  and  sorbite  with  pearlite  or  troostite 
or  both. 


APPENDIX  —  NOMENCLATURE  OF  T1IK  MICROSCOPIC  CONSTITUENTS     465 

than  troostite  or  sorbite,  and  usually  less,  though  sometimes  more,  than  martensite, 
which  is  recognized  by  its  zigzag  shape  and  needle  structure.  With  ferrite  and  pearlite 
it  is  never  associated. 

Physical  Properties.  —  Maurer's  austenite  of  2  per  cent  manganese  plus  2  per 
cent  carbon  is  but  little  harder  than  soft  iron,  and  25  per  cent  nickel  steel  and  Had- 
field's  manganese  steel  are  but  moderately  hard.  Yet  as  usually  preserved  in  hardened 
high  carbon  steel,  the  hardness  of  austenite  does  not  fall  very  far  short  of  that  of  the 
accompanying  martensite,  probably  because  partly  transformed  in  cooling-.  (Os- 
mond's words  are  that  it  is  "considerably"  softer  than  that  martensite.) 

Specific  Magnetism.  —  Very  slight  unless  perhaps  in  intenselieTds.  In  Hadfield's 
manganese  steel  and  25  per  cent  nickel  steel,  very  ductile. 

Illustrations.  —  "Microscopic  Analysis  of  Metals,"  Figures  20,  50,  and  51  on 
pp.  39,  100,  and  101. 

Cementite  (Sorby,  "intensely  hard  compound";  Ger.  Cementit,  Fr.  Cementite; 
Arnold,  crystallized  normal  carbide).  Definite  metaral. 

Definition.  —  Tri-ferrous  carbide,  Fe3C.  The  name  is  extended  by  some  writers 
so  as  to  include  tri-earbides  in  which  part  of  the  iron  is  replaced  by  manganese  or 
other  elements.  Such  carbides  may  be  called  " manganiferous  Cementite,"  etc. 

Occurrence.  —  (a)  Pearlitic  as  a  component  of  pearlite,  q.v.;  (6)  eutectic; 
(c)  primary  or  pro-eutectic;  (d)  pro-eutectoid;  (e)  that  liberated  by  the  splitting  up  of 
the  eutectic  or  of  pearlite;  and  (/)  uncoagulated  in  sorbite,  troostite,  and  perhaps  mar- 
tensite. (c),  (d),  and  (e)  are  grouped  together  as  "free"  or  "massive." 

Primary  cementite  is  generated  in  cooling  through  Region  3;  eutectic  cementite 
on  cooling  past  the  line  EBD;  pro-eutectoid  cementite  in  cooling  through  Region  5; 
pearlitic  cementite  on  cooling  past  the  line  PSK,  or  AI.  Though  the  several  varieties 
of  cementite  are  generally  held  to  be  all  metastable,  tending  to  break  up  into  graphite 
plus  either  austenite  above  AI  or  ferrite  below  A],  yet  they  have  a  considerable  and 
often  great  degree  of  persistence.  The  graphitizing  tendency  is  completely  checked 
in  the  cold  but  increases  with  the  temperature  and  with  the  proportion  of  carbon 
and  of  silicon  present,  and  is  opposed  by  the  presence  of  manganese. 

Crystallization.  —  Orthorhombic,  in  plates. 

Structure.  —  (a)  Pearlitic,  in  parallel  unintersecting  plates  alternating  with  plates 
of  ferrite;  (6)  eutectic,  plates  forming  a  network  filled  with  a  fine  conglomerate 
of  pearlite  with  or  without  pro-eutectoid  cementite;  (c)  primary,  in  manganiferous 
white  cast  iron,  etc.,  in  rhombohedral  plates;  (d)  in  hyper-eutectoid  steel,  pro-eutec- 
toid cementite  forms  primarily  a  network  enclosing  meshes  of  pearlite  through  which 
cementite  plates  or  spines  sometimes  shoot  if  the  network  is  coarse;  (e)  cementite 
liberated  from  pearlite  merges  with  any  neighboring  cementite;  (/)  the  structure  of 
uncoagulated  cementite  cannot  be  made  out.  On  long  heating  the  pro-eutectoid 
and  pearlitic  cementite  spheroidize  slowly,  and  neighboring  particles  merge;  (a)  in 
white  irons  rich  in  phosphorus  in  flat  plates  embedded  in  iron-carbon-phosphorus 
eutectic. 

Etching,  etc.  —  After  polishing  stands  in  relief.  Brilliant  white  after  etching  with 
dilute  hydrochloric  or  picric  acid;  darkened  by  boiling  with  solution  of  sodium  picrate 
in  excess  of  sodium  hydrate. 

Physical  Properties.  —  Hardest  component  of  steel.  Hardness  =  6  of  Mohs  scale. 
Scratches  glass  and  felspar  but  not  quartz;  very  brittle.  Specific  magnetism  about 
two  thirds  that  of  pure  iron. 


460     APPENDIX  —  NOMENCLATURE  OF  THE  MICROSCOPIC  CONSTITUENTS 

Illustrations.  —  "Microscopic  Analysis  of  Metals,"  Figures  42  and  43  on 
pp.  84,  85. 

Martensite  (Fr.  Martensite,  Ger.  Martensit).     Metaral.    Its  nature  is  in  dispute. 

Definition.  —  The  early  stage  in  the  transformation  of  austenite  characterized  by 
needle  structure  and  great  hardness,  as  in  hardened  high-carbon  steel. 

Constitution.  —  I.  (Osmond  and  others.)  A  solid  solution  like  austenite,  q.v.,  ex- 
cept that  the  iron  is  partly  beta,  whence  its  hardness,  and  partly  alpha,  whence  its 
magnetism  in  mild  fields.  II.  (Le  Chatelier.)  The  same  except  that  its  iron  is  essen- 
tially alpha,  and  the  hardness  due  to  the  state  of  solid  solution.  III.  (Arnold.)  A  spe- 
cial structural  condition  of  his  "hardenite"  (austenite);  not  widely  held.  IV.  A  solid 
solution  in  gamma  iron.  V.  (Benedicks.)  The  same  as  I,  except  that  the  iron  is  wholly 
beta  and  that  beta  iron  consists  of  alpha  iron  containing  a  definite  quantity  of  gamma 
iron  in  solution. 

Equilibrium.  —  It  is  not  in  equilibrium  in  any  part  of  the  diagram,  but  represents 
a  metastable  condition  in  which  the  metal  is  caught  during  rapid  cooling,  in  transit 
between  the  austenite  condition  stable  above  the  line  Ai  and  the  condition  of  ferrite 
plus  cementite  into  which  the  steel  habitually  passes  on  cooling  slowly  past  the 
line  AI. 

Occurrence.  —  The  chief  constituent  of  hardened  carbon  tool  steels,  and  of  medium 
nickel  and  manganese  steels.  In  still  less  fully  transformed  steels  (1.50  per  cent 
carbon  steel  rapidly  quenched,  etc.)  it  is  associated  with  austenite;  in  more  fully 
transformed  ones  (lower  carbon  steels  hardened,  high  carbon  steels  oil  hardened,  or 
water  hardened  and  slightly  tempered,  or  hardened  thick  pieces  even  of  high  carbon 
steel)  it  is  associated  with  troo'stite,  and  with  some  pro-eutectoid  ferrite  or  cementite, 
q.v.,  in  hypo-  and  hyper-eutectoid  steels  respectively.  In  tempering  it  first  changes 
into  troostite;  at  350  deg. -400  deg.  it  passes  through  the  stage  of  osmondite;  at 
higher  temperatures  it  changes  into  sorbite;  and  at  700  deg.  into  granular  pearlite. 
On  heating  into  the  transformation  range  this  changes  into  austenite,  which  on  cool- 
ing again  yields  lamellar  pearlite. 

Characteristic  specimens  are  had  by  quenching  bars  1  cm.  square  of  eutectoid 
steel,  i.e.  steel  containing  about  0.9  per  cent  of  carbon,  in  cold  water  from  800  (leg.  C. 
(1472  deg.  F.). 

Structure.  —  When  alone,  habitually  in  flat  plates  made  up  of  intersecting  needles 
parallel  to  the  sides  of  a  triangle.  When  mixed  with  austenite,  zigzag  needles,  lances, 
and  shafts. 

If  produced  by  quenching  after  heating  to  735  deg.  C.,  it  consists  of  minute  crystal- 
lites resembling  the  globulites  of  Vogelsang,  which  are  rarely  arranged  in  triangular 
order.  At  times  so  fine  as  to  suggest  being  amorphous. 

Etching.  —  With  picric  acid,  iodine  or  very  dilute  nitric  acid  etches  usually  darker 
than  austenite,  but  sometimes  lighter,  always  darker  than  ferrite  and  cementite,  but 
always  lighter  than  troostite. 

Illustrations.  —  "Microscopic  Analysis  of  Metals,"  Figure  19  on  p.  38,  Figure  52 
on  p.  102. 

Ferrite    (Fr.  Ferrite,  Ger.  Ferrit).     Definite  metaral. 

Definition.  —  Free  alpha  iron. 

Composition.  —  Nearly  pure  iron.  It  may  contain  a  little  phosphorus  and 
silicon  but  its  carbon  content,  if  any,  is  always  small,  at  the  most  not  more  than  0.05 
per  cent,  and  perhaps  never  as  much  as  0.02  per  cent. 


APPKXDIX  — XOMKXCLA'ITKK  OF  THE  MICROSCOPIC  COXSTITUENTS     467 

Occurrence.  —  (a)  Pearl i tic  as  a  component  of  pearlite,  q.v.;  (b)  pro-eutectoid 
ferrite  generated  in  slow  cooling  through  the  transformation  range;  (c)  that  segre- 
gated from  pearlite,  i.e.  set  free  by  the  splitting  up  of  pearlite,  especially  in  low  car- 
bon steel;  (d)  uncoagulated  as  in  sorbite,  and  probably  troostite.  (b)  and  (c)  are 
classed  together  as  free  or  massive. 

Thus  ferrite  is  normal  and  stable  in  regions  7  and  8. 

Crystallization.  —  Isometric,  in  cubes  or  octahedra. 

Structure.  —  (a)  Pearlitic  ferrite,  unintersecting  parallel  plates  alternating  with 
plates  of  ceincntite;  (b)  pro-eutectoid  ferrite  in  low  carbon  steel  forms  irregular  poly- 
gons, each  with  uniform  internal  orientation.  In  higher  carbon  steel  after  moderately 
slow  cooling,  especially  in  the  presence  of  manganese,  it  forms  a  network  enclosing 
meshes  of  pearlite.  In  slower  cooling  this  network  is  replaced  by  irregular  grains 
separated  by  pearlite;  (c)  the  ferrite  set  free  by  the  splitting  up  of  pearlite  merges 
with  the  pro-eutectoid  ferrite,  if  any;  (d)  the  structure  of  the  ferrite  in  sorbite,  etc., 
cannot  be  made  out. 

Etch  ini/. —  Dilute  alcoholic  nitric  or  picric  acid  on  light  etching  leaves  the 
ferrite  grains  white  with  junctions  which  look  dark.  Deeper  etching,  by  Heyn's 
reagent  or  its  equivalent,  reveals  the  different  orientation  of  the  crystals  or  grains, 
(a)  as  square  figures  parallel  to  the  direction  of  the  etched  surface,  (b)  as  plates  which 
dip  at  varying  angles  and  become  dark  or  bright  when  the  specimen  is  rotated  under 
oblique  illumination.  Still  deeper  etching  reveals  the  component  cubes  (etching 
figures,  Atzfiguren),  at  least  if  the  surface  is  nearly  parallel  to  the  cube  faces.. 

Physical  Properties.  —  Soft;  relatively  weak  (tenacity  about  40,000  Ibs.,  per 
sq.  in.);  very  ductile;  strongly  ferro-magnetic ;  coercitive  force  very  small. 

drain  Xize.  —  For  important  purposes  (1)  etch  deeply  enough,  e.g.  with  copper- 
ammonium  chloride,  to  reveal  clearly  the  junctions  of  the  grains;  (2)  count  on  a  photo- 
graph of  small  magnification  the  number  of  grains  in  a  measured  field  so  drawn  as  to 
exclude  fragments  of  grains;  after  (3)  determining  the  true  grain  boundaries  by  ex- 
amination under  high  powers  (Heyn's  method).  Deep  nitric  acid  etching  is  inaccurate, 
because  an  apparent  grain  boundary  may  contain  several  grains. 

Illustrations.  —   "  Microscopic  Analysis  of  Metals,"  Figures  41,  56  on  pp.  79,  116. 

Osmondite  (Fr.  Osmondite,  Ger.  Osmondit). 

Definition.  —  That  stage  in  the  transformation  of  austenite  at  which  the  solubility 
in  dilute  sulphuric  acid  reaches  its  maximum  rapidity.  Arbitrarily  taken  as  the 
boundary  between  troostite  and  sorbite. 

Earlier  Definition.  —  Denned  by  the  Vth  Congress  as  having  the  "maximum  sol- 
ubility in  acids  and  by  a  maximum  coloration  under  the  action  of  acid  metallographic 
reagents."  The  present  definition  is  confined  to  maximum  rapidity  of  dissolving, 
because  we  do  not  yet  know  that  this  in  all  cases  co-exists  with  the  maximum  depth 
of  coloration,  and  in  any  case  in  which  these  two  should  not  co-exist,  the  old  defini- 
tion does  not  decide  which  is  true  osmondite. 

Constitution. —  The  following  hypotheses  have  been  suggested,  none  of  which 
has  firm  experimental  foundation:  (1)  A  solid  solution  of  carbon  or  an  iron  carbide 
in  alpha  iron.  (2)  The  colloidal  system  of  Benedicks  in  its  purity,  troostite  being  this 
system  while  forming  at  the  expense  of  martensite,  and  sorbite,  being  this  system 
coagulating  and  passing  into  pearlite.  (3)  The  stage  of  maximum  purity  of  amor- 
phous alpha  iron  on  the  way  to  crystallizing  into  ferrite. 

Occurrence.  —  Hardened  carbon  steel  of  about  1  per  cent  of  carbon  when  reheated 


468     APPENDIX  — NOMENCLATURE  OF  THE  MICROSCOPIC  CONSTITUENTS 

(tempered)  to  350-400  deg.  C.  passes  through  the  stage  of  troostite  to  that  of 
osmondite,  and  on  higher  heating  to  that  of  sorhite.  What  variation  if  any  from  this 
temperature  is  needed  to  bring  hardened  steel  of  other  carbon  content  to  the  osmond- 
ite stage  is  not  known.  In  that  it  represents  a  true  boundary  state  between  troostite 
and  sorbite  it  differs  in  meaning  from  troosto-sorbite,  which  embraces  both  the  troost- 
ite and  the  sorbite  which  lie  near  this  boundary.  Indeed  osmondite  has  sometimes 
been  used  in  this  looser  sense.  Writers  are  cautioned  that,  however  useful  these  terms 
may  prove  for  making  these  nice  discriminations,  they  are  not  likely  to  be  familiar 
to  general  readers. 

Etching.  —  According  to  Heyn  it  differs  from  troostite  and  sorbite  in  being  that 
stage  in  tempering  which  colors  darkest  on  etching  with  alcoholic  hydrochloric  acid. 

The  present  definition  and  description  of  osmondite  should  displace  previous  ones, 
because  they  have  the  express  approval  of  Professor  Heyn,  the  proposer  of  the  name, 
and  M.  Osmond  himself. 

Ferronite  (Fr.Ferronite,  Ger.  Ferronit)  (Benedicks).   Hypothetical  definite  metaral. 

Definition.  —  Solid  solution  of  about  0.27  per  cent  of  carbon  in  beta  iron. 

Occurrence  (hypothetical).  —  In  slowly  cooled  steels  and  cast  iron  containing 
0.50  per  cent  of  combined  carbon  or  more,  that  which  is  generally  believed  to  be  fer- 
rite,  whether  pearlitic  or  free,  is  supposed  by  Benedicks  to  be  ferronite. 

Hardenite  (Fr.  Hardenite,  Ger.  Hardenit). 

Definition.  —  Collective  name  for  austenite  and  martensite  of'eutectoid  composi- 
tion. It  includes  such  steel  (1)  when  above  the  transformation  range,  and  (2)  when 
hardened  by  rapid  cooling. 

Observations.  —  On  the  generally  accepted  theory  that  austenite  is  a  solid  solution 
of  carbon  or  an  iron  carbide  in  iron,  hardenite  is  the  solution  of  the  lowest  transforma- 
tion temperature,  i.e.  the  eutectoid.  The  theory  that  instead  it  is  a  definite  chemical 
compound,  Fe2.iC,  is  considered  under  Austenite.  Its  proposer  includes  under 
hardenite  both  eutectoid  (0.90  per.  cent  carbon)  austenite  when  above  the  transforma- 
tion range  and  the  martensite  into  which  that  austenite  shifts  in  rapid  cooling  (hard- 
ening). 

Other  Meanings.  —  Originally  (Howe,  1888)  collective  name  for  austenite  and 
martensite  of  any  composition  in  carbon  steel.  Osmond  (1897),  austenite  saturated 
with  carbon.  Both  these  meanings  are  withdrawn  by  their  proposers. 

Pearlite  (Sorby's  "pearly  constituent."  At  first  written  "pearlyte"  Fr.  Perlite, 
Ger.  Perlit).  Aggregate. 

Definition.  —  The  iron-carbon  eutectoid,  consisting  of  alternate  masses  of  ferrite 
and  cementite. 

Constitution  and  Composition.  —  A  conglomerate  of  about  6  parts  of  ferrite  to  1  of 
cementite.  When  pure,  contains  about  0.90  per  cent  of  carbon,  99.10  per  cent  of 
iron. 

Occurrence.  —  Results  from  the  completion  of  the  transformation  of  austenite 
brought  spontaneously  to  the  eutectoid  carbon  content,  and  hence  occurs  in  all 
carbon  steels  and  cast  iron  containing  combined  carbon  and  cooled  slowly  through 
the  transformation  range,  or  held  at  temperatures  in  or  but  slightly  below  that  range, 
long  enough  to  enable  the  ferrite  and  cementite  to  coagulate  into  a  mass  microscopic- 
ally resoluble.  Hence  it  is  the  normal  constituent  in  Region  8.  Its  ferrite  is  stable 
but  its  cementite  is  metastable.and  tends  to  transform  into  ferrite  and  graphite. 

Varieties  and  Structure.  —  Because  pearlite  is  formed  by  the  coagulation  of  the 


APPENDIX  —  NOMENCLATURE  OF  THE  MICROSCOPIC  CONSTITUENTS    469 

ferrite  and  cementite  initially  formed  as  the  irresoluble  emulsion,  sorbite,  (Arnold's 
sorbitic  pearlite)  there  are  the  indefinitely  bounded  stages  of  sorbitic  pearlite  (Arnold's 
normal  pearlite),  i.e.  barely  resoluble  pearlite,  in  the  border-land  between  sorbite  and 
laminated  pearlite;  granular  pearlite,  in  which  the  cementite  forms  fine  globules  in  a 
matrix  of  ferrite;  and  laminated  or  lamellar  pearlite,  consisting  of  fine,  clearly  defined, 
non-intersecting,  parallel  lamellae  alternately  of  ferrite  and  cementite.  The  name 
granular  pearlite  was  first  used  by  Sauveur  to  represent  what  is  now  called  sorbite. 
This  meaning  has  been  withdrawn. 

An  objection  to  Arnold's  name  "normal  poarlite"  is  that  ii^ is  likely  to  mislead. 
"Normal"  here  apparently  refers  to  arising  under  normal  conditions  of  cooling,  but 
(1)  it  rather  suggests  structure  normal  for  pearlite,  which  surely  is  the  lamination 
characteristic  of  eutectics  in  general,  and  (2)  the  general  reader  has  no  clue  as  to  what 
conditions  of  cooling  are  here  called  normal.  Many  readers  are  not  manufacturers, 
and  even  in  manufacture  itself  air  cooling  is  normal  for  one  branch  and  extremely 
slow  furnace  cooling  for  another.  Arnold  calls  troostite  "troostitic  pearlite"  and 
sorbite  "sorbitic  pearlite."  This  is  contrary  to  general  usage,  which  restricts  pearlite 
to  microscopically  resoluble  masses. 

Etching.  —  After  etching  with  dilute  alcoholic  nitric  or  picric  acid  it  is  darker  than 
ferrite  or  cementite  but  lighter  than  sorbite  and  troostite.  A  magnification  of  at 
least  250  diameters  is  usually  needed  ^or  resolving  it  into  its  lamellae,  though  the 
pearlite  of  blister  steel  can  often  be  resolved  with  a  magnification  of  25  diameters. 
The  more  rapidly  pearlite  is  formed,  the  higher  the  magnification  needed  for  re- 
solving it. 

Illustrations.  —  Lamellar  pearlite.  Osmond  and  Stead,  "Microscopic  Analysis," 
Figure  11  on  p.  19,  Granular  pearlite,  idem,  Figure  18  on  p.  36;  Heyn  and  Bauer, 
"Stahl  und  Eisen,"  1906,  Figure  14,  opposite  p.  785. 

Graphite    (Ger.  Graphit,  Fr.  Graphite).     Definite  metaral. 

Definition.  —  The  free  elemental  carbon  which  occurs  in  iron  and  steel. 

Composition.  —  Probably  pure  carbon,  identical  with  native  graphite. 

Genesis.  —  Derived  in  large  part,  and  according  to  Gcerens  wholly,  from  the  de- 
composition of  solid  cementite.  Others  hold  that  its  formation  as  kish  may  be  from 
solution  in  the  molten  metal,  and  that  part  of  the  formation  of  temper  graphite  may 
be  from  elemental  carbon  dissolved  in  austenite.  It  is  the  stable  form  of  carbon  in 
all  parts  of  the  diagram. 

Occurrence.  —  (1)  as  kish,  flakes  which  rise  to  the  surface  of  molten  cast  iron  and 
usually  escape  thence; 

(2)  as  thin  plates,  usually  curved,  e.g.  in  gray  cast  iron,  representing  carbon  which 
has  separated  during  great  mobility,  i.e.  near  the  melting  range; 

(3)  as  temper  graphite  (Ger.  Temperkohle,  Ledebur)  pulverulent  carbon  which 
separates  from  cementite  and  austenite,  especially  in  the  annealing  process  for  mak- 
ing malleablized  castings. 

Graphite  and  ferrite  are  sometimes  associated  in  a  way  which  suggests  strongly 
that  they  represent  a  graphite-austenite  eutectic.  But  the  existence  of  such  a  true 
eutectic  is  doubted  by  most  writers. 

Properties.  —  Hexagonal.  H.  1-2.  Gr.  2.255.  Streak  black  and  shining,  luster 
metallic;  macroscopic  color,  iron  black  to  dark  steel  gray,  but  always  black  when  seen 
in  polished  sections  of  iron  or  steel  under  the  microscope;  opaque;  sectile;  soils  paper; 
flexible;  feel,  greasy. 


470    APPENDIX  — NOMENCLATURE  OF  Till:  MICROSCOPIC  COXSTlTTKVrs 

Troostite  (Fr.  Troostitc,  Ger.  Troostit).  Probably  agrregate.  (Arnold,  troostitic 
pearlite.) 

Definition.  —  In  the  transformation  of  austenite,  the  stage  following  martensite 
and  preceding  sorbite  (and  osnionditc  if  this  stage  is  recognized). 

<  'oH.fliltilion  an/I  Composition.  —  An  uncoagulated  conglomerate  of  the  transition 
stages.  The  degree  of  completeness  of  the  transformation  represented  by  it  is  not 
definitely  known  and  probably  varies  widely.  Osmond  and  most  others  believe  that 
the  transformation,  while  generally  far  advanced,  yet  falls  materially  short  of  comple- 
tion; but  Benedicks  and  Arnold  (9)  believe  that  it  is  complete.  The  former  belief 
that  it  is  a  definite  phase,  e.g.  a  solid  solution  of  carbon  or  an  iron  carbide  in  either  /3 
or  7  iron,  is-  abandoned.  Its  carbon  content  like  that  of  austenite  and  martensite 
varies  widely. 

Occurrence. — It  arises  either  on  reheating  hardened  (e.g.  martensitie  steel)  to 
slightly  below  400  deg.,  or  on  cooling  through  the  transformation  range  at  an  inter- 
mediate rate,  e.g.  in  small  pieces  of  steel  when  quenched  in  oil,  or  quenched  in  water 
from  the  middle  of  the  transformation  range,  or  in  the  middle  of  larger  pieces  quenched 
in  water  from  above  the  transformation  range.  With  slightly  farther  reheating  it 
changes  into  sorbite;  with  higher  heating  into  sorbitic  pearlite,  then  slowly  into  granular 
pearlite,  and  probably  indirectly  into  lamellar  pearlite.  It  occurs  in  irregular,  fine- 
granular  or  almost  amorphous  areas,  colored  darker  by  the  common  etching  reagents 
than  the  martensite  or  sorbite  accompanying  it.  A  further  common  means  of  dis- 
tinguishing it  from  sorbite  is  that  it  is  habitually  associated  with  martensite,  whereas 
sorbite  is  habitually  associated  with  pearlite. 

Areas  near  the  boundary  between  troostite  and  sorbite  are  sometimes  called 
troosto-sorbite. 

Properties.  —  Hardness,  intermediate  between  that  of  the  martensitic  and  the 
pearlitic  state  corresponding  to  the  carbon  content  of  the  specimen.  In  general  the 
hardness  increases,  the  elastic  limit  rises,  and  the  ductility  decreases,  as  the  carbon 
content  increases.  Its  ductility  is  increased  rapidly  and  its  hardness  and  elastic  limit 
lowered  rapidly  by  further  tempering,  which  affects  it  much  more  markedly  than 
sorbite. 

Sorbite    (Fr.  Sorbite,  Ger.  Sorbit).    Aggregate.    (Arnold,  sorbitic  pearlite.) 

Definition.  —  In  the  transformation  of  austenite,  the  stage  following  troostite 
and  osmondite  if  the  stage  is  recognized,  and  preceding  pearlite. 

Constitution  and  Composition.  —  Most  writers  believe  that  it  is  essentially  an  un- 
coagulated conglomerate  of  irresoluble  pearlite  with  ferrite  in  hypo-  and  cementite 
in  hyper-eutectoid  steels  respectively,  but  that  it  often  contains  some  incompletely 
transformed  matter. 

Occurrence.  —  The  transformation  can  be  brought  to  the  sorbitic  stage  (1)  by  re- 
heating hardened  steel  to  a  little  above  400  deg.,  but  not  to  700  deg.  at  which  tem- 
perature it  coagulates  into  granular  pearlite;  (2)  by  quenching  small  pieces  of  steel 
in  oil  or  molten  lead  or  even  by  air  cooling  them;  (3)  by  quenching  in  water  from  just 
above  the  bottom  of  the  transformation  range,  An.  Sorbite  is  ill-defined,  almost  amor- 
phous, and  is  colored  lighter  than  troostite  but  darker  than  pearlite  by  the  usual 
etching  reagents.  It  differs  further  from  troostite  in  being  softer  for  given  carbon 
content,  and  usually  in  being  associated  with  pearlite  instead  of  martensite,  and 
from  pearlite  in  being  irresoluble  into  separate  particles  of  ferrite  and  cementite. 

As  sorbite  is  essentially  a  mode  of  aggregation  it  cannot  properly  be  represented 


APPENDIX  — NOMENCLATURE  OF  THE  MICROSCOPIC  CONSTITUENTS     471 

on  the  equilibrium  diagram.  Its  components  at  all  times  tend  to  coagulate  into 
pearlite,  yet  it  remains  in  its  uncoagulated  state  at  all  temperatures  below  400  deg. 

Properties.  —  Though  slightly  less  ductile  than  pearlitic  steel  for  given  carbon 
content,  its  tenacity  and  elastic  limit  are  so  high  that  a  higher  combination  of  these 
three  properties  can  be  had  in  sorbitic  than  in  pearlitic  steels  by  selecting  a  carbon 
content  slightly  lower  than  would  be  used  for  a  pearlitic  steel.  Hence  the  use  of 
sorbitic  steels,  e.g.  first  hardened  and  then  annealed  cautiously,  for  structural  pur- 
poses needing  the  best  quality. 

Manganese  Sulphide  (Fr.  Sulphur  de  Manganese,  Ger^  Schwefelmangan),  MnS 
(Arnold  and  Waterhouse).  Metaral. 

Occurrence,  etc.  —  Sulphur  combines  with  the  manganese  present  in  preference  to 
the  iron,  forming  pale  dove  or  slate  gray  masses,  rounded  in  castings,  elongated  in 
forcings. 

Ferrous  Sulphide   (Fr.  Sulphure  de  Fer,  Ger.  Schwefeleisen),  FeS.     Metaral. 

Occurrence.  —  The  sulphur  not  taken  up  by  the  manganese  forms  ferrous  sulphide, 
FeS,  which,  probably  associated  in  part  with  iron  as  an  Fe-FeS  eutectic,  forms  by 
preference  more  or  less  continuous  membranes  surrounding  the  grains  of  pearlite. 
Color,  yellow  or  pale  brown. 

Sulphur  Prints.  —  When  silk  impregnated  with  mercuric  chloride  and  hydrochloric 
acid  (Heyn's  and  Bauer's  method)  or  bromide  paper  moistened  with  sulphuric  acid 
(Baumann's  method)  is  pressed  on  polished  steel,  the  position  of  the  sulphur-bearing 
areas,  whether  of  FeS  or  MnS,  records  itself  by  the  local  blackening  which  the  evolved 
H2S  causes.  Phosphorus  bearing  areas  also  blacken  Baumann's  bromide  paper. 

MISCELLANEOUS 

Eutectoid,  Saturated,  etc.  —  The  iron-carbon  eutectoid  is  pearlite.  Steel  with  more 
carbon  than  pearlite  is  called  hyper-eutectoid,  that  with  less  is  called  hypo-eutectoid. 
Arnold's  names  "saturated,"  "unsaturated,"  and  "supersaturated,"  for  eutectoid, 
hypo-eutectoid,  and  hyper-eutectoid  steel  respectively,  have  considerable  industrial 
vise  in  English-speaking  countries,  but  are  avoided  by  most  scientific  writers  on  the 
ground  that  they  are  misleading,  because,  e.g.  there  is  only  one  specific  temperature, 
AI,  at  which  eutectoid  steel  is  actually  saturated,  and,  if  any  other  temperature  is  in 
mind,  that  steel  is  not  saturated.  Above  AI  it  is  clearly  undersaturated. 

The  objection  to  the  names  sorbite,  troostite,  martensite,  and  austenite,  that 
each  of  them  covers  steel  of  a  wide  range  of  carbon  content,  is  to  be  dismissed  because 
a  like  objection  applies  with  equal  force  to  every  generic  name  in  existence. 


The  theoretical  matter  in  this  report  is  given  solely  for  exposition  and  the  com- 
mittee disclaims  the  intent  to  impose  any  theory.  This  report  is  offered  for  adop- 
tion subject  to  this  disclaimer  on  the  ground  that  the  adoption  of  theories  is  beyond 
the  powers  of  a  Congress. 


INDEX 


A,   AT,  Ac,  Ar3,   Ac3,  Ar3.2,  Acs.2,  Ar3.2.i,  Ac3.2.i,  Arcm,  Accm.    See  critical  points, 

notation. 

Allotropic  theory  of  the  hardening  of  steel,  87,  309 
Allotropy,  definition  of,  106 
of  cementite,  192 

iron,  106 

Alloy  steels.     See  special  steels. 
Alloys,  constitution  of,  407 

,  fusibility  curves  of,  411  to  427 
,  microstructure  of,  411  to  427 

of  iron  and  carbon,  equilibrium  diagram  of,  439  to  448 
,  fusibility  curves  of,  428  to  448 
,  phase  rule  applied  to,  457  to  459 
,  structural  composition  immediately  after  solidification  of, 

429 

,  phase  rule  applied  to,  454  to  459 
,  solidification  of,  409  to  427 
,  structural  composition  of,  421  to  448 
,  whose  component  metals  form  solid  solutions,  solidification  and  constitution 

of,  411  to  415 

are  insoluble  in  each  other  in  the  solid  state,  solid- 
ification and  constitution  of,  415  to  423 
partially  soluble  in  each  other  in  the  solid  state, 
solidification  and  constitution  of,  417  to  448 
Alpha  iron,  182,  207 

,  crystallization  of,  107 
,  description  of,  106 

theory  of  the  hardening  of  steel,  310 
Alumina  powder  for  polishing,  preparation  of,  52,  53 
American  ingot  iron,  101 

Amorphous  cement  vs.  boundaries  of  crystalline  grains,  91,  103 
heat  treatment  of  pure  metals,  97 
the  straining  of  metals,  94 
Anhedrons.     See  allotrimorphic  crystals. 
Annealing,  air  cooling  in,  235 

cold  worked  steel,  246 
,  cooling  in,  233 
,  double  treatment  in,  240 

for  malleablizing  cast  iron,  401 
,  furnace  cooling  in,  235 
,  heating  for,  231 
,  influence  of  maximum  temperature  in,  236 

time  at  maximum  temperature  in,  239 
,  nature  of  operation,  231 

of  steel,  231  to  273 
,  oil  and  water  quenching  in,  239 
'473 


474  INDEX 

Annealing,  purpose  of,  231 

,  rate  of  cooling  vs.  carbon  content  in,  234 
size  of  objects  in,  234 
steel  castings,  248 
temperatures  for  steel,  233 
Austenite,  crystallization  of,  215,  262 

,  definition,  description,  occurrence,  and  structure  of,  277  to  280 
,  Osmond's  test  showing  relative  softness  of,  281 
,  relative  softness  of,  286 
,  saturated,  429 

Austenitic  and  pearlitic  structures,  relation  between,  263 
special  steels,  332 
steel,  tempering  of,  300 

B 

Baumann  on  sulphur  printing,  45 

Beilby  on  amorphous  cement,  91,  94 

Belaiew  on  the  crystallization  of  steel  and  meteorites,  288  to  216 

Benedicks'  equilibrium  diagram  of  iron-carbon  alloys,  446 

Beta  iron,  183 

,  crystallization  of,  107 
,  description  of,  106 

theory  of  the  hardening  of  steel,  309 
Binary  alloys.     See  alloys. 
Bismuth,  crystalline  grains  of,  90 
Bivariant  equilibrium,  definition  of,  454 
Black  heart  castings,  401 

,  annealing  for,  402 
Brittleness,  intercrystalline,  271 
,  intergranular,  271 

of  low  carbon  steel,  271 
Burnt  steel,  production  and  structure  of,  259 

C 

Cameras,  27  to  33 

Campion  and  Ferguson  fusible  alloy,  41 
Carbide  steel,  326 

Carbon,  condition  of,  in  hardened  and  tempered  steel,  305 
,  hardening  and  combined  in  steel,  305 
in  pearlite,  131 
in  steel,  119 
solubility  in  iron,  364 
temper,  398 

theory  of  the  hardening  of  steel,  310 
Carpenter  and  Keeling's  cooling  curves  of  steels,  167 

determinations  of  the  critical  points,  167 
equilibrium  diagram  of  iron-carbon  alloys,  445,  446 
Case  hardened  articles,  heat  treatment  of,  324 

steel,  tempering  of,  325 
hardening  by  gas,  319 

,  composition  of  iron  or  steel  subjected  to,  315 
,  cooling  after,  324 
,  distribution  of  carbon  after,  316 
,  duration  of,  316 
,  materials  used  for,  318 
,  mechansim  of,  322 
of  steel,  315  to  325 


INDEX  475 

Case  hardening,  temperatures  for,  315 

Cast  iron,  calculation  of  structural  composition  of,  370,  375,  394 
,  chilled  castings  of,  378 

,  constitution,  properties,  and  structure  of,  304  to  397 
containing  only  combined  carbon,  369 
graphitic  carbon,  366 
,  eutectic,  379,  381 

,  formation  of  combined  and  graphitic  carbon  in,  366 
,  impurities  in,  386  to  397 
,  influence  and  occurrence  of  manganese  in,  387 

phosphorus  in,  388 
silicon  in,  386 
sulphur  in,  386 
,  malleable,  398-406 
,  solidification  and  cooling  of,  381 
,  structural  composition  vs.  physical  properties  of,  378 
steel,  structure  of,  208  to  220 
Castings  suitable  for  malleablizing,  399 
Cement  carbon,  definition  of,  122 
Cementation.     See  case  hardening. 

'o!  iron  and  steel,  315  to  325 
Cementite,  allotropy  of,  192 

,  definition  and  description  of,  122 

,  etching  of,  131 

,  free,  definition  of,  129 

,  graphitizing  of,  257,  384,  398 

in  high  carbon  steel,  128 
,  primary.     See  cementite,  pro-eutectic. 
,  pro-eutectic,  432 
,  spheroidizing  of,  252 
Cementitic  special  steels,  326,  333 
Chilled  castings,  378 
Chrome-nickel  steel,  353 
steel,  349 

,  uses  and  properties  of,  349 

-tungsten  steel.     See  high-speed  steel. 

Chromium,  influence  on  critical  points  of  iron  of,  349 

Cleavage,  brittlenoss.     See  intercrystalline  brittleness. 

definition  of,  86 
Cold  working,  crystalline  growth  after,  96 

,  influence  on  structure  and  properties  of  steel  of,  227 
Colloidal  solution,  285 
Components,  definition  of,  454 
Condensers,  26,  27 
Cooling  and  heating  curves  of  iron  and  steel,  169  to  181 

curves  of  pure  metals,  407 
Copper,  microstructure  of,  86 

,  twinnings  in,  95 

Critical  points  and  crystallization,  200,  204 
dilatation,  199 
electrical  conductivity,  200 
magnetic  properties,  203 

,  calorimetric  method  for  determination  of,  179 
,  Carpenter  and  Keeling's  determination  of,  167 
,  causes  of,  182  to  198 
,  definition  of,  158 
,  determination  of,  169  to  181 
,  graphical  representation  of  the  position  and  magnitude  of,  169 


476  INDEX 

Critical  points,  heat  absorbed  or  evolved  at,  167 

in  high  carbon  (hyper-eutectoid)  steel,  166 
iron,  description  of,  163 
medium  high  carbon  steel,  165 
pure  iron,  163 
very  low  carbon  steel,  164 

,  influence  of  chemical  composition  on  position  of,  162 
speed  of  heating  and  cooling  on,  161,  162 
,  magnetic  method  for  determination  of,  179 
,  melting-points  method  for  determination  of,  179 
,  merging  of,  165,  167 

,  metallographic  method  for  determination  of,  179 
,  minor,  167 
,  notation,  159 
,  occurrence  of,  159  to  181 
,  relation  between  structure  of  steel  and,  195 
,  their  effects,  199  to  207 

,  thermo-electric  method  for  determination  of,  179 
,  use  of  neutral  bodies  in  detecting,  173 
range.     See  critical  points, 
temperatures.     See  critical  points. 
Crystalline  grains.      See  grains. 

growth  in  metals  on  annealing,  96 

of  strained  ferrite,  265  to  271 
Crystallite  of  iron,  105 
Crystallites,  definition  of,  87 
Crystallization  and  critical  points,  200,  204 
,  cubic,  of  metals,  90 
of  austenite,  208 
iron,  103 
steel,  208 

,  process  of,  86,  87 
Crystallography,  systems  of,  91 
Crystals,  allotrimorphic,  definition  of,  87 
,  cubic,  of  iron,  103 
,  definition  of,  86 
,  formation  of,  86 
,  idiomorphic,  definition  of,  87 
,  mixed.     See  mixed  crystals. 
Cubic  crystallization  of  iron,  103 
metals,  90 


Degrees  of  freedom,  definition  of,  452 

liberty.     See  degrees  of  freedom. 
Desch's  types  of  cooling  curves,  177 
Dilatation  and  critical  points,  199 
Divariant  equilibrium.     See  bivariant  equilibrium. 
Ductility  of  steel,  structural  composition  vs.,  141 

E 

Electric  arc  lamps,  23  to  27 

furnaces,  39 

Electrical  conductivity  and  critical  points,  200 
Electrolytic  iron,  crystallizing  properties  of,  111,  112 

microstructure  of,  101 
Electromagnetic  stages,  15 


INDEX  477 

Equilibrium,  bivariant,  definition  of,  454 
,  definition  of,  452 
diagram.     See  fusibility  curves. 

of  iron-carbon  alloys,  439  to  448 

,  Benedicks'  diagram,  446 
,  Carpenter  and  Keeling's  diagram,  445 
,  Roberts-Austen's  diagrams,  442,  443 
,  Roozeboom's  diagram,  444 
,  Rosenhain's  diagram,  447 
,  Ruff's  diagram,  449 
,  the  author's  early  diagram,  441 
,  Upton's  diagram,  448 
,  Wittorff's  diagram,  450 
,  metastable,  definition  of,  453 
,  stable,  definition  of,  452 
,  univariant,  definition  of,  454 
,  unstable,  definition  of,  453 
,  unvariant,  definition  of,  454 
Ktching,  42,_43,  62,  64 

figures.     See  etching  pits, 
for  macrostructure,  47 
of  cementite,  43,  131 

wrought  iron,  45 
pits,  formation  of,  90 
in  iron,  46,  104 
nitric  acid,  42 
picric  acid,  42 
sodium  picrate,  43 
Stead's  reagent,  43 
Kutcctic  alloys,  99,  120 

,  constitution  and  occurrence  of,  415  to  427 
,  definition  of,  418 
,  cast  iron,  379 
,  iron-carbon,  429 
Eutectoid,  definition  of,  121 

steel,  definition  and  structure  of,  126,  127 
Ewing  and  Rosenhain,  straining  of  iron  by,  110 

metals,  92 
Rosenhain's  theory  of  crystalline  growth  of  metals  on  annealing,  96,  97 

F 
1'errite,  crystalline  growth  of,  265 

,  definition  of,  104 
,  free,  121 
in  cast  iron,  367 

low  carbon  steel,  1 19 
wrought  iron,  114 
Fibers  in  wrought,  iron,  llo 

Finishing  temperatures,  influence  on  the  structure  and  properties  of  steel  of,  223 
Free  cementite,  definition  of,  129 

ferrite,  121 
Furnaces,  electric,  39 
Fusibility  curves  of  alloys,  411  to  427 

iron-carbon  alloys,  428  to  450 

G 

Gamma  iron,  182,  207 

,  crystallization  of,  107 


478  INDEX 

Gamma  iron,  description  of,  106 

,  twinning  in,  107 
Ghost  lines  in  steel,  153  to  157 
Grading  of  steel  vs.  its  carbon  content,  118 
Grain  refining  treatment,  241 
Grains,  crystalline  orientation  of,  90 
,  ferrite,  102 

,  orientation  of,  102 
,  growth  of,  on  annealing,  96 
of  metals,  definition  and  formation  of,  89 

,  heterogeneousness  of,  89 
Graphitic  carbon,  factors  influencing  formation  of,  366 

in  cast  iron,  366 
Graphitizing  of  cementite,  257,  384,  398 

in  malleablizing  cast  iron,  398 
Granulation  of  steel,  209 
Gray  cast  iron,  366 

vs.  malleable  cast  iron,  406 
Guillet's  theory  of  special  steels,  326 

H 

Hadfield  steel..  345 
Hard  castings,  399 

Hardened  and  tempered  steel,  microstructure  of,  304 
Hardening  and  tempering  in  one  operation,  297,  299 
carbon,  definition  of,  305 

theory  of  the  hardening  of  steel,  311 
,  cooling  for,  275 
,  heating  for,  274 
of  steel,  274  to  297 

,  theories  of,  308  to  314 
,  structural  changes  on,  276 

theories  of  the  hardening  of  steel,  classification  of,  308 
Hardenite,  definition,  occurrence,  and  properties  of,  291 
Heat  tinting,  44 

treatment  of  case  hardened  articles,  324 
iron,  influence  of,  110,  111 
metals,  influence  of,  96 

Heating  and  cooling  curves  of  iron  and  steel,  169  to  181 
High-speed  steel,  354  to  363 

,  composition  of,  355 
,  discovery  by  Taylor  and  White  of,  354 
,  etching  of,  47,  361 

,  heating  and  cooling  curves  of,  357  to  361 
,  microstructure  of,  355 
,  properties  of,  354 
,  theory  of,  356 
,  treatment  of,  354 

Hot  working,  influence  on  structure  and  properties  of  steel  of,  221  to  225 
Hyper-eutectoid  steel,  definition  and  structure  of,  127 
Hypo-eutectoid  steel,  definition  and  structure  of,  127 

I 

Idiomorphic  crystals,  definition  of,  87 
Igevsky,  picric  acid  etching,  42 
Illumination  for  microscopical  work,  18  to  27 
Illuminators,  vertical,  20  to  22 
Impurities  in  cast  iron,  386  to  397 


INDEX  479 

Impurities,  in  metals,  influence  of,  97  to  100 
steel,  143  to  157 

,  segregation  of,  153 
influence  on  iron  of,  112 
Ingot  iron,  101 
Ingotism,  220 

Intercrystalline  brittleness,  271 
Intergranular  brittleness,  271 
Interstrain  theory  of  the  hardening  of  steel,  313 
Invar  (nickel  steel),  343 
Inverted  microscope,  31  to  33 
Iris  diaphragms,  11 
Iron,  affinity  for  carbon  of,  315 
,  allotropy  of,  106 
,  alpha,  182,  207 

,  description  of,  108 
,  beta,  182,  207    * 

,  description  of,  108 

-carbon  alloys,  equilibrium  diagram  of,  439  to  450 
,  fusibility  curves  of,  428  to  450 
,  phase  rule  applied  to,  455  to  459 

,  structural  composition  immediately  after  solidification  of,  430 
eutectic,  429 

,  cementation  of,  315  to  325 
-cementite  fusibility  curve.  428 
,  cooling  and  heating  curves  of,  169  to  181 
,  critical  points  of,  163,  182 

crystallite,  104 
,  crystallization  of,  102,  107 
,  crystallizing  of,  111,  112 
,  cubic  crystals  of,  103,  104 
,  electrolytic,  microstructure  of,  101 
,  etching  in  hydrogen,  117 

pits  in,  104 
,  gamma,  182,  207 

,  description  of,  108 
-graphite  fusibility  curve,  434 
,  influence  of  chromium  on  critical  points  of,  349 
heat  treatment  of,  110,  111 
impurities  on,  112 
mechanical  treatment  of,  110 
nickel  on  dilatation  of,  343 
tungsten  on  critical  points  of,  346 
,  microstructure  of,  101 

oxide  in  steel,  153 
,  slip  bands  in,  110 
,  straining  of,  92 

sulphide  in  steel,  146 
Irreversible  steels,  337 
Isomorphous  mixtures,  definition  of,  98 

K 

Kourbatoff's  etching  to  color  cementite,  43 

reagent  for  hardened  steels,  63,  280 

L 


Le  Chatelier's  inverted  microscope,  67  to  71 
Ledebur's  temper  carbon,  398 


480  INDEX 

Lieberkuhn,  20 

Lights  for  microscopical  work,  18  to  27,  50 

Liquidus,  definition  of,  410 

M 

Macrostructure,  etching  for,  47 

Magnetic  properties  and  critical  points,  203 

method  for  determination  of  critical  points,  179 
specimen  holders,  12 
Magnifier,  vertical,  23 
Malleable  cast  iron,  398  to  406 

,  annealing  for  the  manufacture  of,  400 

,  packing  materials  for  the  manufacture  of,  400 

vs.  gray  cast  iron,  406 
castings.     See  malleable  cast  iron. 
Manganese  in  cast  iron,  influence  and  occurrence  of,  387 

steel,  148 
oxide  in  steel,  150 
steel,  343  to  346 
,  austenitic,  345 
,  martensitic,  345 
,  pearlitic,  344 
,  water-toughening  of,  345 
sulphide  in  steel,  145,  151 
Manipulations,  40  to  50 

Martensite,  definition,  description,  occurrence,  properties,  etching,  and  structure  of,  283 
Martensitic  special  steels,  332 

steel,  tempering  of,  302 
Matweieff's  etching  to  color  cementite,  43 

method  of  etching  slag  in  wrought  iron,  1 17 
Maurer,  production  of  austenite  by,  279 
Mechanical  refining,  229 
stages,  8 

treatment  of  iron,  influence  of,  110 
metals,  influence  of,  96 
steel,  221  to  225 
Metallic  alloys.     See  alloys. 

,  constitution  of,  407  to  427 
Metallographic  laboratory,  apparatus  for,  5 
Metallography,  industrial  importance  of,  1 
Metalloscope,  universal,  14  to  18 
Metals,  cooling  curves  of,  407 

,  crystalline  growth  on  annealing,  96 

,  crystallization  of,  87 

,  cubic  crystallization  of,  90 

,  definition  and  formation  of  grains  of,  89 

,  influence  of  heat  treatment,  96,  97 

mechanical  treatment  of,  96 
,  latent  heat  of  solidification  of,  408 
,  phase  rule  applied  to,  455 
,  solidification  of,  407 
Metarals,  definition  of,  293 
Metastable  equilibrium,  definition  of,  453 
Meteorites,  microstructure  of,  212 
Microscopes  and  accessories,  5,  67  to  85 

,  inverted,  31  to  33,  67 
Microstructure  of  cast  steel,  208  to  220 
electrolytic  iron,  101 


INDEX  481 


Microstructure  of  hardened  and  tempered  steel,  304 
high  carbon  steel,  126 
sulphur  steel,  145 
impure  gold,  99 
low  carbon  steel,  119 
medium  high  carbon  steel,  124 
meteorites,  212 
pure  copper,  86,  89 
gold,  86 
iron,  101 
metals,  86 

worked  steel,  221  to  230 
wrought  iron,  113  to  117 
Mixed  crystals,  definition  of,  98 
Mohs  scale  of  hardness,  123 
Molybdenum  steel,  351 

Monovariant  equilibrium.     Sec  univariant  equilibrium 
Mottled  cast  iron,  375 

N 
Nachet  illuminating  objectives,  22 

prism  vertical  illuminator,  21 
Nernst  lamp,  25 
Neumann's  lines,  96,  109 

Neutral  bodies  for  the  detection  of  critical  points,  173 
Nickel,  influence  of,  on  critical  points  of  iron,  337 

dilatation  of  iron,  343 
steel,  336  to  343 
,  austenitic,  342 
,  case  hardening  of,  342 
,  critical  points  of  commercial,  pearlitic,  339 
,  hardening  and  annealing  of,  339 
,  martensitic,  342 
,  pearlitic,  337 
,  properties  of  pearlitic,  338 
Nitric  acid  etching,  42 
Non-variant  equilibrium.     See  unvariant  equilibrium 


O 

Objectives,  8 

Orientation  of  crystalline  grains,  definition  of,  90 

ferrite  grains,  102 
Osmond,'  and  Cartaud  on  the  crystallization  of  iron,  107,  108 

,  on  polishing,  51,  61 

,  production  of  austciiite  by,  277 

Osmondite,  definition,  description,  and  occurrence  of,  303 
Overheating,  258  , 

P 

Parabolic  reflector,  20 
Patented  wire,  247 
Patenting,  247 
Pearlite,  carbon  content  of,  131 

,  definition  and  description  of,  120 
,  formation  of,  190 
in  high  carbon  steel,  126 
low  carbon  steel,  120 
,  varieties  of,  256 


482  INDEX 

Pearlitie  special  steels,  331 
Phase  rule  applied  to  alloys,  454 

iron-carbon  alloys,  457 
metals,  455 

,  definition  of,  454 

,  enunciation  and  explanation  of,  452 
Polymorphism,  1()ti 
Phosphorus  in  cast  iron,  influence  and  occurrence  of,  388 

steel,  144 

Photography.     See  photomicrography. 
Photomicrographic  cameras,  27  to  33 
Photomicrography,  48 
Picrate  of  sodium  etching,  43 
Picric  acid  etching.  42 
Pits.     See  etching  pits. 
Planes  of  cleavage.     See  cleavage. 
Platinite  (nickel  steel),  343 
Point  of  recalescence.     See  recalescence  point. 
Polishing,  40,  41,  51  to  62 

machines,  34,  35,  51  to  61 
Polyhedral  special  steels,  326,  332 
Polymorphism.     See  allotropy. 
Portevin's  etching  for  macrostructure,  47 
Potentiometer,  36 
Preserving  samples.  64 
Prism  vertical  illuminator,  21,  84 
Pseudomorphism,  definition  of,  304 
Pure  metals,  crystallization  and  microstructure  of,  86 

Pyrometer,  Lc  Chatelier  thermo-electric,  for  the  determination  of  the  critical  points,  36 
Pyrometers,  36 

,  self-recording,  37  to  39,  178 

Q 

Quaternary  steels,  335.     See  also  special  steels, 

vanadium  steels,  340 
Quenching  in  annealing,  240 


Recalescence  point,  description  and  occurrence  of,  158 

Red-shortness  in  steel  caused  by  sulphur,  147 

Refining,  mechanical,  229 

Retardations.     See  critical  points. 

Retention  theories  of  the  hardening  of  steel,  308 

Reversible  steels,  337 

Revillon  on  preparation  of  alumina  for  polishing,  53 

Roberts-Austen's  equilibrium  diagrams  of  iron-carbon  alloys.  444 

use  of  neutral  bodies  for  detecting  critical  points,  39,  173 
Robin, 'on  preparation  of  alumina  for  polishing,  53 

production  of  austenite  by,  278 
Rohl  on  sulphur  printing,  45 

Roozeboom's  equilibrium  diagram  of  iron-carbon  alloys,  444 
Rosenhain  and  Ewen,  on  amorphous  cement,  91 

Ewing.     See  Ewing  and  Rosenhain. 

Humfrey,  straining  of  iron  by,  110 

Rosenhain's  equilibrium  diagram  of  iron-carbon  alloys,  447 
Ruff's  equilibrium  diagram  of  iron-carbon  alloys,  449 


INDEX  483 


Saladin  self-recording  pyrometer,  37  to  39 
Saladin's  cooling  and  healing  curves  of  steels,  175 
Segregation  of  impurities  in  steel,  153 
Self-hardening  steel,  348 

-recording  pyrometers,  37  to  39,  178 
Silicates  in  steel,  150 
Silicon  in  cast  iron,  influence  and  occurrence  of,  :>sii 

steel,  143,  144 
steel,  352 
Slag  in  wrought  iron,  114,  110,  117 

,  composition  of,  11G 

Matweieff's  method  of  etching,  117 
,  microstructurc  of,  116 
Slip  bands,  description  and  production  of,  92 

in  iron,  110 

Sodium  picrate  etching,  43 
Solid  solutions,  409 

,  definition  of,  98 
Solidus,  definition  of,  410 
Solution  theories  of  the  hardening  of  steel,  309 
Sonims,  143.  150 

Sorbitc,  definition,  description,  and  formation  of,  225,  230,  289 
Sorby-Beck  parabolic  reflector.  20 
Special  steels,  326  to  363 

,  austcnitic,  332 
,  cementitic,  333 
,  constitution,  properties,  treatment,  and  uses  of  most  important  types, 

320  to  335 

,  definition  and  general  character  of,  326  to  335 
,  influence  of  special  elements  on  position  of  critical  range  in,  328 
,  martensitic,  332 
.  pearlitie,  331 
,  polyhedral,  332 
,  treatment  of,  333 
Specimen  holders,  11  to  11 
Spheroidizing  of  cementite,  252 
Stable  equilibrium,  definition  of,  453 
Stages,  electromagnetic,  15 
,  leveling  60 
,  mechanical,  8 

Stead  and  Carpenter  on  the  crystallizing  properties  of  electrolytic  iron,  111,  112 
on  heat-tinting,  44 

phosphorus  in  cast  iron,  388  to  392 
the  briulcness  of  low  carbon  steel,  271 

crystalline  growth  of  very  low  carbon  steel,  265 
on  polishing,  53  to  55 

Steadite,  definition  and  description  of,  390 
Stead's  brittlcness,  273 

etching  reagent  for  detection  of  phosphorus,  43 
Steel,  annealing  of,  231  to  273 

,  temperatures  of,  232 
,  brittlenesis  of  low  carbon,  '271 
,  burning  of,  259 

,  calculation  of  structural  composition  of,  132,  148 
,  carbon  in,  119 
,  case  hardening  of,  315  to  325 


484  INDEX 

Steel  castings,  annealing  of,  248 

,  causes  of  critical  points  in,  182  to  198 

,  cementation  of,  315  to  325 

,  chemical  tests  for  the  detection  of  sulphur  in,  44,  45,  148 

vs.  structural  composition  of,  134,  148 
,  chrome,  349 

-nickel,  353 
,  constitution,  properties,  treatment,  and  uses  of  most  important  types  of  special, 

33(i  to  363 

,  cooling  and  heating  curves  of,  169  to  181 
,  critical  points  of,  158  to  207 
,  crystallization  of,  208 
,  ductility  vs.  structural  composition  of,  141 
,  effects  of  critical  points  in,  199  to  207 
,  eutectoid,  definition  and  structure  of,  126,  127,  216 
,  formation  of  graphite  in  high  carbon,  257 
,  ghost  lines  in,  153  to  157 
,  grading  of,  118 
,  hardening  of,  274  to  297 
,  high  carbon,  cementite  in,  127  to  131 

,  microstructure  of,  126  to  131 
,  pearlite  in,  126 
-speed,  354  to  363 

,  etching  of,  47 

,  hyper-eutectoid,  definition  and  structure  of,  127,  218 
,  hypo-eutectoid,  definition  and  structure  of,  129,  216 
,  impurities  in,  143  to  157 
,  influence  of  cold  working  on  the  structure  and  properties  of,  227 

finishing  temperatures  on  the  structure  and  properties  of,  223,  229 
hot  working  on  the  structure  and  properties  of,  221 
,  iron  oxide  in,  153 

sulphide  in,  146 
,  irreversible,  337 

,  ferrite  in,  119 

,  microstructure  of,  119  to  121 

,  pearlite  in,  120 

vs.  wrought  iron,  118 
,  manganese,  343  to  346 
in,  148 
oxide  in,  152 
sulphide  in,  145,  151 
,  maximum  strength  of,  140 
,  mechanical  refining  of,  229 

treatment  of,  221  to  225 
,  medium  high  carbon,  microstructure  of,  124,  125 

,  micro-test  for  determination  of  carbon  in,  135 
,  pearlite  in,  124 
,  nickel,  336  to  343 
,  normal  structure  of,  118 
,  occurrence  of  critical  points  in,  158  to  181 
of  maximum  hardening  power,  296 
overheated,  259 
phosphorus  in,  144 

,  physical  properties  of  constituents  of,  137 
,  production  and  structure  of  burnt,  259 
,  relation  between  structure  above  and  below  the  critical  range  of,  262 

and  critical  points  of,  195 
,  reversible,  337 


INDEX  485 

Steel,  segregation  of  impurities  in,  153  to  157 
,  self-hardening,  34-S 
,  silicates  in,  150 
,  silicon,  352 

in,  142,  143 
,  special,  326  to  363 
,  structural  changes  on  cooling  in,  187 

composition  of,  132,  133 
,  structure  of  cast,  208  to  220 

worked,  221  to  230 
,  sulphur  in,  145,  151 
,  tempering  of  hardened,  298  to  306 
,  tenacity  vs.  structural  composition  of,  138  to  140 
,  theories  of  hardening  of,  308  to  314 
,  tungsten,  346 
,  vanadium,  349 

vs.  carbon  content,  grading  of,  118 
,  \\idmanstatten  structure  in,  212 
Straining  of  iron,  92,  110 

,  crystalline  growth  after,  96 
of  metals,  92,  94 

Stress  theories  of  the  hardening  of  steel,  311 
Structural  composition  of  alloys,  221  to  226 

cast  iron,  calculation  of,  370,  375,  395 
iron-carbon  alloys  immediately  after  solidification,  430 
steel,  calculation  of,  92,  132  to  134' 
•Subcarbide  theory  of  the  hardening  of  steel,  311 
Sulphur  in  cast  iron,  influence  and  occurrence  of,  388 
steel,  145 

,  chemical  tests  for  the  detection  of,  44,  45,  151 
printing,  44,  45,  148 


Taylor  and  White's  discovery  of  high-speed  steel,  354 

Temper  carbon,  398 

Temperatures  for  annealing  steel,  232 

Tempering  and  the  retention  theories  of  the  hardening  of  steel,  313 

stress  theory  of  the  hardening  of  steel,  313 
colors,  298 

,  decrease  of  hardness  on,  306 
,  explanation  of,  298 
,  heat  liberated  on,  307 
,  influence  of  rate  of  cooling  in,  299 

time  in,  299 
of  austenitic  steel,  300 
case  hardened  steel,  325 
hardened  steel,  298  to  306 
martensitic  steel,  302 
troostitic  steel,  302 
temperatures,  298 

Tenacity  of  steel  vs.  structural  composition,  138  to  140 
Ternary  steels,  326.     See  also  special  steels. 
Thermal  critical  points.     Sec  critical  points. 

treatment.     See  heat  treatment. 
Toughening  treatment,  241 
Transformation  points.     See  critical  points, 
range.     See  critical  points. 


486  INDEX 

Transition  const  it  uents.     See  also  martensite,  troostite,  and  sorbite. 

,  definition  and  formation  of,  293 

Troostite,  definition,  description,  occurrence,  properties,  etching,  and  structure  of,  285 
Troostitic  steel,  tempering  of,  302 
Troosto-sorbite,  290 
Tschernoff  iron  crystallite,  105 
Tungsten,  influence  on  the  critical  points  of  iron  of,  346 

steel,  :;iii 

Twinnings,  and  amorphous  iron  theory,  313 
definition  of,  95 
in  copper,  95 

gamma  iron,  107 
marble,  94 
produced  by  pressure,  95 

>  U 

Univariant  equilibrium,  definition  of,  454 

Universal  metalloscope,  14  to  18 

Unstable  equilibrium,  definition  of,  453 

Unvariant  equilibrium,  definition  of,  454 

Upton's  equilibrium  diagram  of  iron-carbon  alloys,  448 


Vanadium  steel,  349,  353 
Vertical  illuminators,  18  to  23 
magnifier,  23 


W 


Water-toughening  of  manganese  steel,  345 

Welsbach  lamp,  23 

Widmanstatten  structure,  212 

Wittorff's  equilibrium  diagram  of  iron-carbon  alloys,  450 

White,  cast  iron,  369 

heart  castings,  401 

,  annealing  for,  401 
Maunsel.     See  Taylor  and  White. 
Workshop  microscopes,  81  to  84 
Wrought  iron,  composition  of,  113 
,  definition  of,  113 
,  etching  of,  45 
,  fibers  in,  115 

,  microstructure  of,  114  to  116 
,  slag  in,  114,  116,  117 
vs.  low  carbon  steel,  118 


Y 

Yatscviteh,  M.,  etching  of  high-speed  steel,  47 

Z 

Zciss  prism  illuminator,  21 


••••• 


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